STANMORE OBSERVATORY.
INSIDE VIEW.

TELESCOPIC WORK
FOR
STARLIGHT EVENINGS.

BY
WILLIAM F. DENNING, F.R.A.S.
(FORMERLY PRESIDENT OF THE LIVERPOOL ASTRONOMICAL SOCIETY).

“To ask or search I blame thee not, for heaven

Is as the book of God before thee set,

Wherein to read his wondrous works.”

Milton.

LONDON:
TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
1891.

[All rights reserved.]

PRINTED BY TAYLOR AND FRANCIS,
RED LION COURT, FLEET STREET.

[PREFACE.]

It having been suggested by some kind friends that a series of articles on “Telescopes and Telescopic Work,” which I wrote for the ‘Journal of the Liverpool Astronomical Society’ in 1887-8, should be reprinted, I have undertaken the revision and rearrangement of the papers alluded to. Certain other contributions on “Large and Small Telescopes,” “Planetary Observations,” and kindred subjects, which I furnished to ‘The Observatory’ and other scientific serials from time to time, have also been included, and the material so much altered and extended that it may be regarded as virtually new matter. The work has outgrown my original intention, but it proved so engrossing that it was found difficult to ensure greater brevity.

The combination of different papers has possibly had the effect of rendering the book more popular in some parts than in others. This is not altogether unintentional, for the aim has been to make the work intelligible to general readers, while also containing facts and figures useful to amateur astronomers. It is merely intended as a contribution to popular astronomy, and asserts no rivalry with existing works, many of which are essentially different in plan. If any excuse were, however, needed for the issue of this volume it might be found in the rapid progress of astronomy, which requires that new or revised works should be published at short intervals in order to represent existing knowledge.

The methods explained are approximate, and technical points have been avoided with the view to engage the interest of beginners who may find it the stepping-stone to more advanced works and to more precise methods. The object will be realized if observers derive any encouragement from its descriptions or value from its references, and the author sincerely hopes that not a few of his readers will experience the same degree of pleasure in observation as he has done during many years.

No matter how humble the observer, or how paltry the telescope, astronomy is capable of furnishing an endless store of delight to its adherents. Its influences are elevating, and many of its features possess the charms of novelty as well as mystery. Whoever contemplates the heavens with the right spirit reaps both pleasure and profit, and many amateurs find a welcome relaxation to the cares of business in the companionship of their telescopes on “starlight evenings.”

The title chosen is not, perhaps, a comprehensive one, but it covers most of the ground, and no apology need be offered for dealing with one or two important objects not strictly within its scope.

For many of the illustrations I must express my indebtedness to the Editors of the ‘Observatory’ to the Council of the R.A.S., to the proprietors of ‘Nature,’ to Messrs. Browning, Calver, Cooke & Sons, Elger, Gore, Horne Thornthwaite and Wood, Klein, and other friends.

The markings on Venus and Jupiter as represented on pages 150 and 180 have come out much darker than was intended, but these illustrations may have some value as showing the position and form of the features delineated. It is difficult to reproduce delicate planetary markings in precisely the same characters as they are displayed in a good telescope. The apparent orbits of the satellites of the planets, delineated in figs. 41, 44, &c., are liable to changes depending on their variable position relatively to the Earth, and the diagrams are merely intended to give a good idea of these satellite systems.

W. F. D.

Bishopston, Bristol,
1891.

Plates I. and II. are views of the Observatory and Instruments recently erected by Mr. Klein at Stanmore, Middlesex, lat. 51° 36′ 57″ N., long. 0° 18′ 22″ W. The height above sea-level is 262 feet. The telescope is a 20-inch reflector by Calver, of 92 inches focus; the tube is, however, 152 inches long so as to cut off all extraneous rays. It is mounted equatoreally, and is provided with a finder of 6 inches aperture—one of Tulley’s famous instruments a century ago. The large telescope is fixed on a pillar of masonry 37 feet high, and weighing 115 tons. Mr. Klein proposes to devote the resources of his establishment to astronomical photography, and it has been provided with all the best appliances for this purpose. The observatory is connected by telephone with Mr. Klein’s private residence, and the timepieces and recording instruments are all electrically connected with a centre of observation in his study.

CONTENTS.

TELESCOPIC WORK
FOR
STARLIGHT EVENINGS.

[CHAPTER I.]
THE TELESCOPE, ITS INVENTION AND THE DEVELOPMENT OF ITS POWERS.

The instrument which has so vastly extended our knowledge of the Universe, which has enabled us to acquire observations of remarkable precision, and supplied the materials for many sublime speculations in Astronomy, was invented early in the seventeenth century. Apart from its special application as a means of exploring the heavens with a capacity that is truly marvellous, it is a construction which has also been utilized in certain other departments with signal success. It provided mankind with a medium through which to penetrate far beyond the reach of natural vision, and to grasp objects and phenomena which had either eluded detection altogether or had only been seen in dim and uncertain characters. It has also proved a very efficient instrument for various minor purposes of instruction and recreation. The invention of the telescope formed a new era in astronomy; and though, with a few exceptions, men were slow at first in availing themselves of its far-seeing resources, scepticism was soon swept aside and its value became widely acknowledged.

But though the telescope was destined to effect work of the utmost import, and to reach a very high degree of excellence in after times, the result was achieved gradually. Step by step its powers were enlarged and its qualities perfected, and thus the stream of astronomical discovery has been enabled to flow on, stimulated by every increase in its capacity.

There is some question as to whom may be justly credited with the discovery of its principles of construction. Huygens, in his ‘Dioptrics,’ remarks:—“I should have no hesitation in placing above all the rest of mankind the individual who, solely by his own reflections, without the aid of any fortuitous circumstances, should have achieved the invention of the telescope.” There is reason to conclude, however, that its discovery resulted from accident rather than from theory. It is commonly supposed that Galileo Galilei is entitled to precedence; but there is strong evidence to show that he had been anticipated. In any case it must be admitted that Galilei[1] had priority in successfully utilizing its resources as a means of observational discovery; for he it was who, first of all men, saw Jupiter’s satellites, the crescent form of Venus, the mountains and craters on the Moon, and announced them to an incredible world.

It has been supposed, and not without some basis of probability, that a similar instrument to the telescope had been employed by the ancients; for certain statements contained in old historical records would suggest that the Greek philosophers had some means of extending their knowledge further than that permitted by the naked eye. Democritus remarked that the Galaxy or “Milky Way” was nothing but an assemblage of minute stars; and it has been asked, How could he have derived this information but by instrumental aid? It is very probable he gained the knowledge by inferences having their source in close observation; for anyone who attentively studies the face of the sky must be naturally led to conclude that the appearance of the “Milky Way” is induced by immense and irregular clusterings of small stars. In certain regions of the heavens there are clear indications of this: the eye is enabled to glimpse some of the individual star-points, and to observe how they blend and associate with the denser aggregations which give rise to the milky whiteness of the Galaxy.

Refracting lenses, or “burning-glasses,” were known at a very early period. A lens, roughly figured into a convex shape and obviously intended for magnifying objects, has been recovered from the ruins of Herculaneum, buried in the ejections from Vesuvius in the year 79 A.D. Pliny and others refer to lenses that burnt by refraction, and describe globules of glass or crystal which, when exposed in the sun, transmit sufficient heat to ignite combustible material. The ancients undoubtedly used tubes in the conduct of their observations, but no lenses seem to have been employed with them, and their only utility consisted in the fact of their shutting out the extraneous rays of light. But spectacles were certainly known at an early period. Concave emeralds are said to have been employed by Nero in witnessing the combats of the gladiators, and they appear to have been the same in effect as the spectacles worn by short-sighted people in our own times. But the ancients supposed that the emerald possessed inherent qualities specially helpful to vision, rather than that its utility resulted simply from its concavity of figure. In the 13th century spectacles were more generally worn, and the theory of their construction understood.

It is remarkable that the telescope did not come into use until so long afterwards. Vague references were made to such an instrument, or rather suggestions as to the possibilities of its construction, which show that, although the principle had perhaps been conceived, the idea was not successfully put into practice. Roger Bacon, who flourished in the 13th century, wrote in his ‘Opus Majus’:—“Greater things may be performed by refracted light, for, from the foregoing principles, follows easily that the greatest objects may be seen very small, the remote very near, and vice versâ. For we can give transparent bodies such form and position with respect to the eye and the object that the rays are refracted and bent to where we like, so that we, under any angle, see the objects near or far, and in that manner we can, at a great distance, read the smallest letters, and we can count atoms and sand-grains, on account of the greatness of the angle under which they are seen.”

Fracastor, in a work published at Venice in 1538, states:—“If we look through two eye-lenses, placed the one upon the other, everything will appear larger and nearer.” He also says:—“There are made certain eye-lenses of such a thickness that if the moon or any other celestial body is viewed through them they appear to be so near that their distance does not exceed that of the steeples of public buildings.”

In other writings will also be found intimations as to the important action of lenses; and it is hardly accountable that a matter so valuable in its bearings was allowed to remain without practical issues. The progressive tendency and the faculty of invention must indeed have been in an incipient stage, and contrasts strongly with the singular avidity with which ideas are seized upon and realized in our own day.

Many important discoveries have resulted from pure accident; and it has been stated that the first bonâ fide telescope had its origin in the following incident:—The children of a spectacle-maker, Zachariah Jansen, of Middleberg, in Zealand, were playing with some lenses, and it chanced that they arranged two of them in such manner that, to their astonishment, the weathercock of an adjoining church appeared much enlarged and more distinct. Having mentioned the curious fact to their father, he immediately turned it to account, and, by fixing two lenses on a board, produced the first telescope!

This view of the case is, however, a very doubtful one, and the invention may with far greater probability be attributed to Hans Lippersheim in 1608. Galilei has little claim to be considered in this relation; for he admitted that in 1609 the news reached him that a Dutchman had devised an appliance capable of showing distant objects with remarkable clearness. He thereupon set to work and experimented with so much aptitude on the principles involved that he very soon produced a telescope for himself. With this instrument he detected the four satellites of Jupiter in 1610, and other successes shortly followed. Being naturally gratified with the improvements he had effected in its construction, and with the wonderful discoveries he had made by its use, we can almost excuse the enthusiasm which prompted him to attribute the invention to his own ingenuity. But while according him the honour due to his sagacity in devoting this instrument to such excellent work, we must not overlook the fact that his claim to priority cannot be justified. Indeed, that Galilei had usurped the title of inventor is mentioned in letters which passed between the scientific men of that time. Fuccari, writing to Kepler, says:—“Galileo wants to be considered the inventor of the telescope, though he, as well as I and others, first saw the telescope which a certain Dutchman first brought with him to Venice, and although he has only improved it very little.”

In a critical article by Dr. Doberck[2], in which this letter is quoted and the whole question reviewed with considerable care, it is stated that Hans Lippersheim (also known as Jan Lapprey), who was born in Wesel, but afterwards settled at Middleberg, in the Netherlands, as a spectacle-maker, was really the first to make a telescope, and the following facts are quoted in confirmation:—“He solicited the States, as early as the 2nd October, 1608, for a patent for thirty years, or an annual pension for life, for the instrument he had invented, promising then only to construct such instruments for the Government. After inviting the inventor to improve the instrument and alter it so that they could look through it with both eyes at the same time, the States determined, on the 4th October, that from every province one deputy should be elected to try the apparatus and make terms with him concerning the price. This committee declared on the 6th October that it found the invention useful for the country, and had offered the inventor 900 florins for the instrument. He had at first asked 3000 florins for three instruments of rock-crystal. He was then ordered to deliver the instrument within a certain time, and the patent was promised him on condition that he kept the invention secret. Lapprey delivered the instrument in due time. He had arranged it for both eyes, and it was found satisfactory; but they forced him, against the agreement, to deliver two other telescopes for the same money, and refused the patent because it was evident that already several others had learned about the invention.”

The material from which the glasses were figured appears to have been quartz; and efforts were made to keep the invention a profound secret, as it was thought it would prove valuable for “strategetical purposes.” The cost of these primitive binoculars was about £75 each.

It is singular that, after being allowed to rest so long, the idea of telescopic construction should have been carried into effect by several persons almost simultaneously, and that doubts and disputes arose as to precedence. The probable explanation is that to one individual only priority was really due, but that, owing to the delays, the secret could not be altogether concealed from two or three others who recognized the importance of the discovery and at once entered into competition with the original inventor. Each of these fashioned his instrument in a slightly different manner, though the principle was similar in all; and having in a great measure to rely upon his individual faculties in completing the task, he considered himself in the light of an inventor and put forth claims accordingly. Not only were attempts made to assume the position of inventor, but there arose fraudulent claimants to some of the discoveries which the instrument effected in the hands of Galilei. Simon Marius, himself one of the very first to construct a telescope and apply it to the examination of the heavenly bodies, asserted that he had seen the satellites of Jupiter on December 29, 1609, a few days before Galilei, who first glimpsed them on January 7, 1610. Humboldt, in his ‘Physical Description of the Heavens,’ definitely ascribes the discovery of these moons to Marius; but other authorities uniformly reject the statement, and accord to Galilei the full credit.

It is stated that Galilei’s first instrument magnified only three times, but he so far managed to amplify its resources that he was ultimately enabled to apply a power of 30. The lenses consisted of a double-convex object-glass, and a small double-concave eye-glass placed in front of the focal image formed by the object-glass. The ordinary opera-glass is constructed on a similar principle.

Fig. 1.

The Galilean Telescope.

The discoveries which Galilei effected with this crude and defective instrument caused a great sensation at the time. He made them known through the medium of a publication which he issued under the title of ‘Nuncius Siderus,’ or ‘The Messenger of the Stars.’ In that superstitious age great ignorance prevailed, bigotry was dominant, and erroneous views of the solar system were upheld and taught by authority. We can therefore readily conceive that Galilei’s discoveries, and the direct inferences he put upon them, being held antagonistic to the ruling doctrines, would be received with incredulity and opposition. His views were regarded as heretical. In consequence of upholding the Copernican system he suffered persecution, and had to resort to artifice in the publication of his works. But the marvels revealed by his telescope, though discredited at first, could not fail to meet with final acceptance, for undeniable testimony to their reality was soon forthcoming. They were not, however, regarded until long afterwards as affirming the views enunciated by their clever author. Ultimately the new astronomy, based on the irrepressible evidence of the telescope, and clad in all the habiliments of truth, took the place of the old fallacious beliefs, to form an enduring monument to Copernicus and Galilei, who spent their lives in advancing its cause.

No special developments in the construction of the telescope appear to have taken place until nearly half a century subsequent to its invention. Kepler suggested an instrument formed of two convex lenses, and Scheiner and Huygens made telescopes on this principle in the middle of the 17th century. Huygens found great advantage in the employment of a compound eyepiece consisting of two convex lenses, which corrected the spherical aberration, and, besides being achromatic, gave a much larger field than the single lens. This eyepiece, known as the “Huygenian,” still finds favour with the makers of telescopes.

Fig. 2.

Royal Observatory, Greenwich, in Flamsteed’s time[3].

Huygens may be said to have inaugurated the era of long telescopes. He erected instruments of 12 and 23 feet, having an aperture of 2-1/3 inches and powers of 48, 50, and 92. He afterwards produced one 123 feet in focal length and 6 inches in aperture. Chief among his discoveries were the largest satellite of Saturn (Titan) and the true form of Saturn’s ring. Hevelius of Dantzic built an instrument 150 feet long, which he fixed to a mast 90 feet in height, and regulated by ropes and pulleys. Cassini, at the Observatory at Paris, had telescopes by Campani of 86, 100, and 136 French feet in length; but the highest powers he used on these instruments do not appear to have exceeded 150 times. He made such good use of them as to discover three of the satellites of Saturn and the black division in the ring of that planet. The largest object-glasses employed by Hevelius and Cassini were of 6, 7, and 8 inches diameter. This was during the latter half of the 17th century. In 1712 Bradley made observations of Venus, and obtained measures of the planet’s diameter, with a telescope no less than 212 feet in focal length. The instruments alluded to were manipulated with extreme difficulty, and observations had to be conducted in a manner very trying to the observer. Tubes were sometimes dispensed with, the object-glass being fixed to a pole and its position controlled by various contrivances—the observer being so far off, however, that he required the services of a good lantern in order to distinguish it!

The immoderate lengths of refracting-telescopes were necessary, as partially avoiding the effects of chromatic aberration occasioned by the different refrangibility of the seven coloured rays which collectively make white light. In other words, the coloured rays having various indices of refraction cannot be brought to a coincident focus by transmission through a single lens. Thus the red rays have a longer focus than the violet rays, and the immediate effect of the different refractions becomes apparent in the telescopic images, which are fringed with colour and not sharply defined. High magnifying powers serve to intensify the obstacle alluded to, and thus the old observers found it imperative to employ eye-glasses not beyond a certain degree of convexity. The great focal lengths of their object-lenses enabled moderate power to be obtained, though the eye-glass itself had a focus of several inches and magnified very little.

Sir Isaac Newton made many experiments upon colours, and endeavoured to obviate the difficulties of chromatic aberration, but erroneously concluded that it was not feasible. He could devise no means to correct that dispersion of colour which, in the telescopes of his day, so greatly detracted from their effectiveness. His failure seems to have had a prejudicial effect in delaying the solution of the difficulty, which was not accomplished until many years afterwards.

Fig. 3.

Sir Isaac Newton[4].

Fig. 4.

Gregorian Telescope.

The idea of reflecting-telescopes received mention as early as 1639; but it was not until 1663 that Gregory described the instrument, formed of concave mirrors, which still bears his name. He was not, however, proficient in mechanics, and after some futile attempts to carry his theory into effect the exertion was relinquished. In 1673 Cassegrain revived the subject, and proposed a modification of the form previously indicated by Gregory. Instead of the small concave mirror, he substituted a convex mirror placed nearer the speculum; and this arrangement, though it made the telescope shorter, had the disadvantage of displaying objects in an inverted position. But the utility of these instruments was not demonstrated in a practical form until 1674, when Hooke, the clever mechanician, gave his attention to the subject and constructed the first one that was made of the kind.

Fig. 5.

Cassegrainian Telescope.

In the meantime (1672) Sir Isaac Newton had completed with his own hands a reflecting-telescope of another pattern. In this the rays from the large concave speculum were received by a small plane mirror fixed centrally at the other end of the tube, and inclined at an angle of 45°; so that the image was directed at right angles through an opening in the side, and there magnified by the eye-lens. But for a long period little progress was effected in regard to reflecting-telescopes, owing to the difficulty of procuring metal well adapted for the making of specula.

Fig. 6.

Newtonian Telescope.

In 1729 Mr. Chester Moor Hall applied himself to the study of refracting-telescopes and discovered that, by a combination of different glasses, the colouring of the images might be eliminated. It is stated that Mr. Hall made several achromatic glasses in 1733. A quarter of a century after this John Dollond independently arrived at the same result, and took out a patent for achromatic telescopes. He found, by experiments with prisms, that crown and flint glass operated unequally in regard to the divergency of colours induced by refraction; and, applying the principle further, he obtained a virtually colourless telescope by assorting a convex crown lens with a concave flint lens as the object-glass. Dollond also made many instruments having triple object-lenses, and in these it was supposed that previous defects were altogether obliterated. Two convex lenses of crown glass were combined with a concave lens of flint glass placed between them.

Whether we regard Hall or Dollond as entitled to the most praise in connection with this important advance, it is certain that it was one the value of which could hardly be overestimated. It may be said to have formed a new era in practical astronomy. Instruments only 4 or 5 feet long could now be made equally if not more effective than those of 123 and 150 feet previously used by Huygens and Hevelius. All the troubles incidental to these long unmanageable machines now disappeared, and astronomers were at once provided with a handy little telescope capable of the finest performances.

Fig. 7.

Common Refracting-Telescope.

Reflecting-telescopes also underwent marked improvements in the eighteenth century. Short, the optician, who died in 1768, was deservedly celebrated for the excellent instruments he made of the Gregorian form. Towards the latter part of the century William Herschel, by indomitable perseverance, figured a considerable number of specula. Some of these were mounted as Newtonians; others were employed in the form known as the “Front view,” in which a second mirror is dispensed with altogether, and the rays from the large concave speculum are thrown to the side of the tube and direct to the eyepiece. This construction is often mentioned as the “Herschelian,” but the idea had long before been detailed by Le Maire. In 1728 he presented a paper to the Académie des Sciences, giving his plans for a new reflecting-telescope. He proposed to suppress the small flat speculum in Newtonians, and “by giving the large concave speculum a little inclination, he threw the image, formed in its focus, to one side of the tube, where, an eye-glass magnifying it, the observer viewed it, his back at the time being turned towards the object in the heavens; thus the light lost in the Newtonian telescope by the second reflexion was saved.”

Fig. 8.

The Le Mairean or Herschelian Telescope.

After making several instruments of from 18 to 24 inches aperture, Herschel began one of larger calibre, and it was finished on August 28, 1789. The occasion was rendered historical by the discovery of one of the faintest interior satellites of Saturn, Enceladus. The large telescope had a speculum 48 inches in diameter; the tube was made of rolled or sheet iron, and it was 39 ft. 4 in. long and 4 ft. 10 in. in diameter. It was by far the largest instrument the world had seen up to that time; but it cannot be said to have realized the expectations formed of its powers, for its defining properties were evidently not on a par with its space-penetrating power. Many of Herschel’s best observations were made with much smaller instruments. The large telescope, which was mounted in Herschel’s garden at Slough, soon fell into comparative disuse, and, regarding it as incapable of further usefulness, Sir John Herschel sealed it up on January 1, 1840.

During the next half-century we hear of no attempts being made to surpass the large instrument which formed one of the working-tools of Herschel. Then, however, Lord Rosse entered the field, and in the ‘Philosophical Transactions’ for 1840 described a reflector of 3-feet diameter which he had set up at his residence at Parsonstown, Ireland. In 1845 the same nobleman, distinguished alike for his scientific attainments as for his generosity and urbanity of disposition, erected another telescope, the large speculum of which was 6 feet in diameter, 5½ inches in thickness, and its weight 3 tons. Lord Rosse subsequently cast a duplicate speculum of 6 feet and weighing 4 tons. In point of dimensions this instrument far exceeded that of Herschel, and it is still in use, retaining its character as the largest, though certainly not the best, telescope in existence. Its tube is made of 1-inch deal, well bound together with iron hoops; it is 56 feet long and 7 feet in diameter.

Mr. Lassell soon afterwards made large specula. He erected one of 2-feet aperture and 20-feet focus at his residence at Starfield, near Liverpool, and in 1861 mounted one of 4-feet diameter and 37-feet focus. This instrument was for some time usefully employed by him at Malta. After Mr. Lassell’s return to England his great telescope remained in a dismantled state for several years, and ultimately the speculum was broken up and “consigned to the crucible of the bell-founder.”

It is not a little remarkable that Herschel, Rosse, and Lassell personally superintended and assisted in the construction of the monster instruments with which their names are so honourably associated.

In or about the year 1867 a telescope of the Cassegrainian form, and having a metallic speculum 4 feet in diameter and 28-feet focus, was completed by Grubb of Dublin for the observatory at Melbourne. This instrument, which cost something like £14,000, was found defective at first, though the fault does not appear to have rested with the optician.

Up to this period specula were formed of a metal in which copper and tin were largely represented. But the days of metal specula were numbered. Leon Foucault, in the year 1859, published a valuable memoir in which he described the various ingenious methods he employed in figuring surfaces of glass to the required curve. He furnished data for determining accuracy of figure. Formerly opticians had considerable trouble in deciding the quality of their newly-ground specula or object-glasses. They found it expedient to mount them temporarily, and then, by actual trial on difficult objects, to judge of their efficiency. This involved labour and occasioned delay, especially in the case of large instruments. Foucault showed that crucial tests might be applied in the workshop, and that glasses could be turned out of hand without any misgivings as to their perfection of figure.

Foucault’s early experiments in parabolizing glass led him to important results. By depositing a thin coating of silver on his specula he obtained a reflective power far surpassing that of metal. Thereafter metal was not thought of as a suitable material for reflecting-telescopes. Silver-on-glass mirrors immediately came into great request. The latter undoubtedly possess a great superiority over metal, especially as regards light-grasping power, the relative capacity according to Sir J. Herschel being as ·824 to ·436. Glass mirrors have also another advantage in being less heavy than those of metal. It is true the silver film is not very durable, but it can be renewed at any time with little trouble or expense.

With of Hereford, and after him Calver of Chelmsford, became noted for the excellency of their glass mirrors. They were found nearly comparable to refractors of the same aperture.

A tendency of the times was evidently in the direction of large instruments. One of 47·2-inches aperture (for which a sum of 190,000 francs was paid) was completed by Martin in 1875 for the Paris Observatory, but its employment since that year has not furnished a very successful record. The largest instrument of the kind yet made has a speculum 5 feet in diameter and 27½-feet focal length. It was placed in position in September 1888, and was made by the owner, Mr. Common, of Ealing, whose previous instrument was a 37-inch glass reflector by Calver. The 5-foot telescope is undoubtedly of much greater capacity than the colossal reflector of Lord Rosse, though it is not so large.

Mr. Calver has recently figured a 50-inch mirror for Sir H. Bessemer, but the mounting is not completed; and he is expecting to make other large reflectors, viz. one over 5 feet in diameter and another over 3 feet. The late Mr. Nasmyth also erected some fine instruments, and adopted a combination of the Cassegrainian and Newtonian forms to ensure greater convenience for the observer. Instead of permitting the rays from the small convex mirror to return through the large mirror, he diverted them through the side of the tube by means of a flat mirror, as in Newtonians. But this construction is not to be commended, because much light is lost and defects increased by the additional mirror.

Smaller telescopes of the kind we have been referring to have become extremely popular: and deservedly so. They are likely to maintain their character in future years; for the Newtonian form of instrument, besides being thoroughly effective in critical work, is moderate in price and gives images absolutely achromatic. Moreover, it is used with a facility and ease which an experienced observer knows how to appreciate. Whatever may be the altitude of the objects under scrutiny, he is enabled to retain a perfectly convenient and natural posture, and may pursue his work during long intervals without any of the fatigue or discomfort incidental to the use of certain other forms of instrument.

Returning now to refractors: many years elapsed after Dollond patented his achromatic object-glass before it was found feasible to construct these instruments of a size sufficient to grasp faint and delicate objects. Opticians were thwarted in their efforts to obtain glass of the requisite purity for lenses, unless in small disks very few inches in diameter. It is related that Dollond met with a pot of uncommonly pure flint glass in 1760, but even with this advantage of material he admitted that, after numerous attempts, he could not provide really excellent object-glasses of more than 3-3/4-inches diameter. It may therefore be readily imagined that a refractor of 4½ or 5-inches aperture was an instrument of great rarity and expense. Towards the latter part of the 18th century Tulley’s price was £275 for a 5-inch equatoreally mounted.

Fig. 9.

10-inch Reflecting-Telescope on a German Equatoreal, by Calver.

In later years marked improvements were effected in the manufacture of glass. A sign of this is apparent in the fact that, in 1829, Sir James South was enabled to purchase a 12-inch lens. Four years before this the Dorpat telescope, having an objective of 9½ inches, had created quite a sensation. As time went on, still larger glasses were made. In 1862 Alvan Clark & Sons, of New York, U.S.A., finished an instrument of 18½-inches aperture, at a cost of £3700; and in 1869 Cooke & Sons mounted a 24·6-inch object-glass for the late Mr. Newall, of Gateshead. The latter instrument was much larger than any other refractor hitherto made, but it was not long to maintain supremacy. One of 25·8 inches and 29-feet focus was finished in 1872 by Alvan Clark & Sons for the Naval Observatory, Washington, at a cost of £9000. Another, of similar size, was supplied by the same firm to Mr. McCormick, U.S.A. Several important discoveries, including the satellites of Mars, were effected with the great Washington telescope. A few years later a 27-inch was completed by Grubb for the Vienna Observatory, and quite recently the four largest refractors ever made have been placed in position and are actively employed in various departments of work. These include a 29-inch by Martin for the Paris Observatory, a 30-inch by Henry Bros. for Nice, a 30-inch by A. Clark & Sons for Pulkowa, and a 36-inch, also by A. Clark & Sons, for the Lick Observatory on Mount Hamilton in California. The latter has no rival in point of size, though rumours are current that still larger lenses are in contemplation. The tube of the 36-inch is 56 feet long and 3½ feet in diameter at the ends, but the diameter is greater in the middle. It is placed within a great dome 75 feet in diameter. The expense of the entire apparatus is given as follows:—Cost of the dome, $56,850; of the visual objective, $53,000; of the photographic objective, $13,000; of the mounting, $42,000. Total, $164,850. This noble instrument—due to the munificence of one individual, the late Mr. James Lick, of Chicago, who bequeathed $700,000 for the purpose—may be regarded as the king of refracting-telescopes. Placed on the summit of Mount Hamilton, where the atmosphere is exceptionally favourable for celestial observations, and utilized as its resources are by some of the best observers in America, we may confidently expect it to largely augment our knowledge of the heavenly bodies.

The great development in the powers of both refracting and reflecting-telescopes, as a means of astronomical discovery, exemplifies in a remarkable degree the ever-increasing resources and refinements of mechanical art. In 1610 Galilei, from his window at Padua, first viewed the moon and planets with his crude instrument having a power of 3, and he achieved much during the remaining years he lived, by increasing it tenfold, so that at last he could magnify an object 30 times. Huygens laboured well in the same field; and others who succeeded him formed links in the chain of progress which has almost uninterruptedly run through all the years separating Galilei’s time from our own. The primitive efforts of the Florentine philosopher appear to have had their sequel in the magnificent telescope which has lately been erected under the pure sky of Mount Hamilton. The capacity of this instrument relatively to that of earlier ones may be judged from the fact that a power of about 3300 times has lately been employed with success in the measurement of a close and difficult double star. Could Galilei but stand for a few moments at the eyepiece of this great refractor, and contemplate the same objects which he saw, nearly three centuries ago, through his imperfect little glasses at Padua, he would be appalled at the splendid achievements of modern science.

[CHAPTER II.]
RELATIVE MERITS OF LARGE AND SMALL TELESCOPES.

The number of large telescopes having so greatly increased in recent years, and there being every prospect that the demand for such instruments will continue, it may be well to consider their advantages as compared with those of much inferior size. Object-glasses and specula will probably soon be made of a diameter not hitherto attained; for it is palpably one of the ambitions of the age to surpass all previous efforts in the way of telescopic construction. There are some who doubt that such enormous instruments are really necessary, and question whether the results obtained with them are sufficient return for the great expense involved in their erection. Large instruments require large observatories; and the latter must be at some distance from a town, and in a locality where the atmosphere is favourable. Nothing can be done with great aperture in the presence of smoke and other vapours, which, as they cross the field, become ruinous to definition. Moreover, a big instrument is not to be manipulated with the same facility as a small one: and when anything goes wrong with it, its rectification may be a serious matter, owing to the size. Such telescopes need constant attention if they would be kept in thorough working order. On the other hand, small instruments involve little outlay, they are very portable, and require little space. They may be employed in or out of doors, according to the inclination and convenience of the observer. They are controlled with the greatest ease, and seldom get out of adjustment. They are less susceptible to atmospheric influences than larger instruments, and hence may be used more frequently with success and at places by no means favourably situated in this respect. Finally, their defining powers are of such excellent character as to compensate in a measure for feeble illumination.

In discussing this question it will be advisable to glance at the performances of certain instruments of considerable size.

The introduction of really large glasses dates from a century ago, when Sir W. Herschel mounted his reflector, 4 feet in aperture, at Slough. He discovered two of the inner satellites of Saturn very soon after it was completed; but apart from this the instrument seems to have achieved little. Herschel remarked that on August 28, 1789, when he brought the great instrument to the parallel of Saturn, he saw the spots upon the planet better than he had ever seen them before. The night was probably an exceptionally good one, for we do not find this praise reiterated. Indeed, Herschel appears to have practically discarded his large instrument for others of less size. He found that with his small specula of 7-ft. focus and 6·3-in. aperture he had “light sufficient to see the belts of Saturn completely well, and that here the maximum of distinctness might be much easier obtained than where large apertures are concerned.” Even in his sweeps for nebulæ he employed a speculum of 20-ft. focus and 18½-in. aperture in preference to his 4-ft. instrument, though on objects of this nature light-grasping power is essentially necessary. The labour and loss of time involved in controlling the large telescope probably led to its being laid aside for more ready means, though Herschel was not the man to spare trouble when an object was to be gained. His life was spent in gleaning new facts from the sky; and had the 4-foot served his purpose better than smaller instruments, no trifling obstacle would have deterred him from its constant employment. But his aim was to accomplish as much as possible in every available hour when the stars were shining, and experience doubtless taught him to rely chiefly upon his smaller appliances as being the most serviceable. The Le Mairean form, or “Front view,” which Herschel adopted for the large instrument may quite possibly have been in some degree responsible for its bad definition.

Fig. 10.

Lord Rosse’s 6-foot Reflecting-Telescope.

Lord Rosse’s 6-ft. reflector has now been used for nearly half a century, and its results ought to furnish us with good evidence as to the value of such instruments. It has done important work on the nebulæ, especially in the re-observation of the objects in Sir J. Herschel’s Catalogues of 1833 and 1864. To this instrument is due the discovery of spiral nebulæ; and perhaps this achievement is its best. But when we reflect on the length of its service, we are led to wonder that so little has been accomplished. For thirty years the satellites of Mars eluded its grasp, and then fell a prize to one of the large American telescopes. The bright planets[5] have been sometimes submitted to its powers, and careful drawings executed by good observers; but they show no extent of detail beyond what may be discerned in a small telescope. This does not necessarily impugn the figure of the large speculum, the performance of which is entirely dependent upon the condition of the air. The late Dr. Robinson, of Armagh, who had the direction of the instrument for sometime, wrote in 1871:—“A stream of heated air passing before the telescope, the agitation and hygrometric state of the atmosphere, and any differences of temperature between the speculum and the air in the tube are all capable of injuring or even destroying definition, though the speculum were absolutely perfect. The effect of these disturbances is, in reflectors, as the cube of their apertures; and hence there are few hours in the year when the 6-foot can display its full powers.” Another of the regular observers, Mr. G. J. Stoney, wrote in 1878:—“The usual appearance [of the double star γ² Andromedæ] with the best mirrors was a single bright mass of blue light some seconds in diameter and boiling violently.” On the best nights, however, “the disturbance of the air would seem now and then suddenly to cease for perhaps half a second, and the star would then instantly become two very minute round specks of white light, with an interval between which, from recollection, I would estimate as equal to the diameter of either of them, or perhaps slightly less. The instrument would have furnished this appearance uninterruptedly if the state of the air had permitted.” The present observer in charge, Dr. Boeddicker, wrote the author in 1889:—“There can be no doubt that on favourable nights the definition of the 6-foot is equal to that of any instrument, as is fully shown by Dr. Copeland’s drawings of Jupiter published in the ‘Monthly Notices’ for March 1874. It appears to me, however, that the advantage in going from the 3-foot to the 6-foot is not so great in the case of planets as in the case of nebulæ; yet, as to the Moon, the detail revealed by the 6-foot on a first-class night is simply astounding. The large telescope is a Newtonian mounted on a universal joint. For the outlying portions of the great drawing of the Orion nebula it was used as a Herschelian. As to powers profitably to be used, I find no advantage in going beyond 600; yet formerly on short occasions (not longer than perhaps 1 hour a night) very much higher powers (over 1000) have been successfully employed by my predecessors.”

Mr. Lassell’s 4-foot reflector was taken to Malta, and while there its owner, assisted by Mr. Marth, discovered a large number of nebulæ with it, but it appears to have done nothing else. His 2-foot reflector, which he had employed in previous years, seems to have been his most effective instrument; for with this he discovered Ariel and Umbriel, the two inner satellites of Uranus, Hyperion, the faintest satellite of Saturn, and the only known satellite of Neptune. He also was one of the first to distinguish the crape ring of Saturn. Mr. Lassell had many years of experience in the use of large reflectors; and in 1871 he wrote:—“There are formidable and, I fear, insurmountable difficulties attending the construction of telescopes of large size.... These are, primarily, the errors and disturbances of the atmosphere and the flexure of the object-glasses or specula. The visible errors of the atmosphere are, I believe, generally in proportion to the aperture of the telescope.... Up to the size [referring to an 8-in. O.-G.] in question, seasons of tranquil sky may be found when its errors are scarcely appreciable; but when we go much beyond this limit (say to 2 feet and upwards), both these difficulties become truly formidable. It is true that the defect of flexure may be in some degree eliminated, but that of atmospheric disturbance is quite unassailable. These circumstances will always make large telescopes proportionately less powerful than smaller ones; but notwithstanding these disadvantages they will, on some heavenly objects, reveal more than any small ones can.” Mr. Lassell’s last sentence refers to “delineations of the forms of the fainter nebulæ,” to “seeing the inner satellites of Uranus, the satellite of Neptune, and the seventh satellite of Saturn.” He mentions that, when at Malta, he “saw, in the 2-foot equatoreal, with a power of 1027, the two components of γ² Andromedæ distinctly separated to the distance of a neat diameter of the smaller one. Now, no telescope of anything like 8-inches diameter could exhibit the star in this style.”

The large Cooke refractor of 24·8-inches aperture, which has been mounted for about twenty years at Gateshead, has a singularly barren record. Its atmospheric surroundings appear to have rendered it impotent. The owner of this fine and costly instrument wrote the author in 1885:—“Atmosphere has an immense deal to do with definition. I have only had one fine night since 1870! I then saw what I have never seen since.”

The Melbourne reflector of 4-feet aperture performed very indifferently for some years, and little work was accomplished with it. Latterly its performance has been more satisfactory; excellent photographs of the Moon have been taken, and it has been much employed in observations of nebulæ. The speculum having recently become tarnished, it has been dismounted for the purpose of being repolished.

The silver-on-glass reflector of 47·2-in. diameter, at the Paris Observatory, was used for some years by M. Wolf, who has also had the control of smaller telescopes. He was in a favourable position to judge of their relative effectiveness. In a lecture delivered at the Sardonne on March 6, 1886, he said:—“During the years I have observed with the great Parisian telescope I have found but one solitary night when the mirror was perfect.” Further on, he adds:—“I have observed a great deal with the two instruments [both reflectors] of 15·7 inches and 47·2 inches. I have rarely found any advantage in using the larger one when the object was sufficiently luminous.” M. Wolf also avers that a refractor of 15 inches or reflector of 15·7 inches will show everything in the heavens that can be discovered by instruments of very large aperture. He always found a telescope of 15·7-inch aperture surpass one of 7·9 inches, but expresses himself confidently that beyond about 15 inches increased aperture is no gain.

The Washington refractor of 25·8 inches effected a splendid success in Prof. Hall’s hands in 1877, when it revealed the two satellites of Mars. But immediately afterwards these minute bodies were shown in much smaller instruments; whence it became obvious that their original discovery was not entirely due to the grasp of the 25·8-inch telescope, but in a measure to the astuteness displayed by Prof. Hall in the search. A good observer had been associated with a good telescope; and an inviting research having been undertaken, it produced the natural result—an important success. The same instrument, in the same hands, enabled the rotation-period of Saturn to be accurately determined by means of a white spot visible in December 1876 on the disk of the planet, and which was subsequently seen by other observers with smaller glasses. Good work in other directions has also been accomplished at Washington, especially in observations of double stars and faint satellites. But notwithstanding these excellent performances, Prof. Hall expressed himself in rather disparaging terms of his appliances, saying “the large telescope does not show enough detail.” He gave a more favourable report in 1888; for we find it stated that “the objective retains its figure and polish well. By comparison with several other objectives which Prof. Hall has had an opportunity of seeing during recent years, he finds that the glass is an excellent one.”

Prof. Young, who has charge of the 23-inch refractor at Princeton, has also commented on the subject of the definition of large telescopes. He says:—“The greater susceptibility of large instruments to atmospheric disturbances is most sadly true; and yet, on the whole, I find also true what Mr. Clark told me would be the case on first mounting our 23-inch instrument, that I can almost always see with the 23-inch everything I see with the 9½-inch under the same atmospheric conditions, and see it better,—if the seeing is bad only a little better, if good immensely better.” Prof. Young also mentioned that a power of 1200 on the 23-inch “worked perfectly on Jupiter on two different evenings in the spring of 1885 in bringing out fine details relating to the red spot and showing the true forms of certain white dots on the S. polar belt.”

The 26-inch refractor at the Leander McCormick Observatory, U. S. A., is successfully engaged in observations of nebulæ, and many new objects of this character have been found. It does not appear that the telescope is much used for other purposes; so that we can attach no significance to the fact that important discoveries have not been made with it in other departments.

The great Vienna refractor of 27-inches aperture “does not seem to accomplish quite what was expected of it,” according to Mr. Sawerthal, who recently visited the Observatory at Währing, Vienna. The Director, Dr. Weiss, states in his last report that “the 27-inch Grubb refractor has only been occasionally used, when the objects were too faint for the handier instruments.”

The still larger telescopes erected at the Observatories at Pulkowa and Nice have so recently come into employment that it would be premature to judge of their performance. In the Annual Report from Pulkowa (1887) it is stated that Dr. H. Struve was using the 30-inch refractor “in measuring those of Burnham’s double stars which are only seldom measurable with the ‘old 15-inch,’ together with other stars of which measures are scarce. He made 460 measures in eight or nine months, as well as 166 micro metric observations of the fainter satellites of Saturn and 15 of that of Neptune.” At Nice the 30-inch refractor was employed by M. Perrotin in physical observations of Mars in May and June 1888. The canal-shaped markings of Schiaparelli were confirmed, and some of them were traced “from the ocean of the southern hemisphere right across both continents and seas up to the north polar ice-cap.” The 30-inch also showed some remarkable changes in the markings; but these were not confirmed at other observatories. The telescope evidently revealed a considerable amount of detail on this planet; whence we may infer that its defining power is highly satisfactory.

The great Lick refractor, which appears to have been “first directed to the heavens from its permanent home on Mount Hamilton on the evening of January 3, 1888,” has been found ample work by the zealous astronomers who have it in charge. Prof. Holden, in speaking of it, says:—“It needs peculiar conditions, but when all the conditions are favourable its performance is superb.” Mr. Keeler, one of the observers, writes that, on January 7, 1888, when Saturn was examined, “he not only shone with the brilliancy due to the great size of the objective, but the minutest details of his surface were visible with wonderful distinctness. The outlines of the rings were very sharply defined with a power of 1000.” Mr. Keeler adds:—“According to my experience, there is a direct gain in power with increase of aperture. The 12-inch equatoreal brings to view objects entirely beyond the reach of the 6½-inch telescope, and details almost beyond perception with the 12-inch are visible at a glance with the 36-inch equatoreal. The great telescope is equal in defining power to the smaller ones.” This is no small praise, and it must have been extremely gratifying, not only to those who were immediately associated with the construction of the telescope, but to astronomers everywhere who were hoping to hear a satisfactory report. In its practical results this instrument has not yet, it is true, given us a discovery of any magnitude. It has disclosed several very small stars in the trapezium of the Orion nebula, some difficult double stars have been found and measured, and some interesting work has been done on the planets and nebulæ. Physical details have been observed in the ring nebula, between β and γ Lyræ, which no other telescope has ever reached before.

Mr. Common’s 5-foot reflector has been employed on several objects. In the spring of 1889 Uranus was frequently observed with it, and several minute points of light, suspected to be new satellites, were picked up. Evidence was obtained of a new satellite between Titania and Umbriel; but bad weather and haze, combined with the low altitude of Uranus, interfered with the complete success of the observations. “With only moderate powers, Uranus does not show a perfectly sharp disk. No markings are visible on it, and nothing like a ring has been seen round it.” Mr. Common, in a letter to the writer, dated November 9, 1889, says:—“The 5-foot has only been tried in an unfinished state as yet, the mirror not being quite finished when put into the tube last year. This was in order to gain experience and save the season. It performed much better than I had hoped, and is greatly superior to the 3-foot. I took some very fine photographs with it last year. It has been refigured, or rather completed, this summer, and has just been resilvered.” From this it is evident that Mr. Common’s large instrument has not yet been fully tested; but it clearly gives promise of successful results, and encourages the hope that it will exert an influence on the progress of astronomy. Owing to the highly reflective quality of silvered glass, the 5-foot speculum has a far greater command of light (space-penetrating power) than the great objective mounted at the Lick Observatory. Mr. Common’s mirror may therefore be expected to grasp nebulæ, stars, satellites, and comets which are of the last degree of faintness and quite invisible in the Lick refractor. But we must not forget that the latter instrument is certainly placed in a better atmosphere, and that its action is not therefore arrested in nearly the same degree by haze and undulations of the air. With equal conditions, the great reflector at Ealing would probably far surpass the large refractor we have referred to, the latter having less than one third of the light-grasping power of the former.

This rapid sketch of the performances of some of our finest telescopes must suffice for the present in assisting us to estimate their value as instruments of discovery. And it must be admitted that, on the whole, these appliances have been disappointing. The record of their successes is by no means an extended one, and in some individual cases absolute failure is unmistakable. We must judge of large glasses by their revelations; their capacity must be estimated by results. We often meet with glowing descriptions of colossal telescopes: their advantages are specified and their performances extolled to such a degree that expectation is raised to the highest pitch. But it is not always that such praise is justified by facts. The fruit of their employment is rarely prolific to the extent anticipated, because the observers have been defeated in their efforts by impediments which inseparably attend the use of such huge constructions.

Our atmosphere is always in a state of unrest. Its condition is subject to many variations. Heat, radiated or evolved from terrestrial objects, rises in waves and floats along with the wind. These vapours exercise a property of refraction, with the result that, as they pass in front of celestial objects, the latter at once become subject to a rapid series of contortions in detail. Their outlines appear tremulous, and all the features are involved in a rippling effect that seriously compromises the definition. Delicate markings are quite effaced on a disk which is thus in a state of ebullition; and on such occasions observers are rarely able to attain their ends. Telescopic work is, in fact, best deferred until a time when the air has become more tranquil. In large instruments these disturbances are very troublesome, as they increase proportionately with aperture. They are so pronounced and so persistent as to practically annul the advantage of considerable light-grasping power; for unless the images are fairly well defined, mere brightness counts for nothing. Reflectors are peculiarly susceptible to this obstacle; moreover, the open tube, the fact that rays from an object pass twice through its length, and that a certain amount of heat radiated from the observer must travel across the mouth of the tube all serve to impair the definition. A speculum, to act well, must be of coincident temperature in every part. This is not always the case, owing to the variableness of the weather or to unequal exposure of the speculum. Large refractors, though decidedly less liable to atmospheric influences, are yet so much at the mercy of them that one of the first and most important things discussed in regard to a new instrument is that of a desirable site for it.

The great weight of large objectives and specula tends to endanger the perfect consistency and durableness of their figure, and imposes a severe strain upon their cellular mounting. The glasses must obviously assume a variety of bearings during active employment. This introduces a possible cause of defective performance; for in some instances definition has been found unequal, according to the position of the glass. Specula are very likely to be affected in this manner, as they are loosely deposited in their cells to allow of expansion, and the adjustment is easily deranged. The slightest flaw in the mounting of objectives immediately makes itself apparent in faulty images. Special precautions are of course taken to prevent flexure and other errors of the kind alluded to, and modern adaptations may be said to have nearly eliminated them; but there is always a little outstanding danger, from the ease with which glasses may be distorted or their adjustment become unsettled.

Another difficulty formerly urged against telescopes of great size was the trouble of managing them; but this objection can scarcely be applied to the fine instruments of the present day, which are so contrived as to be nearly as tractable as small ones. A century ago, glass of the requisite purity for large objectives could not be obtained; but this difficulty appears also to have quite disappeared. And the process of figuring lenses of considerable diameter is now effected with the same confidence and success as that of greatly inferior sizes.

Let us now turn for a moment to the consideration of small instruments, premising that in this category are included all those up to about 12-inches aperture. Modern advances have quite altered our ideas as to what may be regarded as large and small telescopes. Sixty-five years ago the Dorpat refractor, with a 9½-inch objective by Fraunhofer, was considered a prodigy of its class; now it occupies a very minor place relatively to the 30-inch and 36-inch objectives at Nice, Pulkowa, and Mount Hamilton.

Prof. Hall remarked, in 1885:—“There is too much scepticism on the part of those who are observing with large instruments in regard to what can be seen with small ones.” This is undoubtedly true; but a mere prejudice or opinion of this sort cannot affect the question we are discussing, as it is one essentially relying upon facts.

Small instruments have done a vast amount of useful work in every field of astronomical observation. Even in the realm of nebulæ, which, more than any other, requires great penetrating power, D’Arrest showed what could be effected with small aperture. Burnham, with only a 6-inch refractor, has equally distinguished himself in another branch; for he has discovered more double stars than any previous observer. Dawes was one of the most successful amateurs of his day, though his instrumental means never exceeded an 8-inch glass. But we need not particularize further. It will be best to get a general result from the collective evidence of past years. We find that nearly all the comets, planetoids, double stars, &c. owe their first detection to comparatively small instruments. Our knowledge of sun-spots, lunar and planetary features is also very largely derived from similar sources. There is no department but what is indebted more or less to the services of small telescopes: the good work they have done is due to their excellent defining powers and to the facility with which they may be used.

Fig. 11.

Refracting-Telescope, by Browning.

We have already said that the record of discoveries made with really large instruments is limited; but it should also be remarked that until quite recently the number of such instruments has been very small. And not always, perhaps, have the best men had the control of them. Virtually the observer himself constitutes the most important part of his telescope: it is useless having a glass of great capacity at one end of a tube, and a man of small capacity at the other. Two different observers essentially alter the character of an instrument, according to their individual skill in utilizing its powers.

Large telescopes are invariably constructed for the special purpose of discovering unknown orbs and gleaning new facts from the firmament. But in attempting to carry out this design, obstacles of a grave nature confront the observer. The comparatively tranquil and sharply definite images seen in small instruments disappear, and in their places forms are presented much more brilliant and expansive, it is true, but involved in glare and subject to constant agitation, which serve to obliterate most of the details. The observer becomes conscious that what he has gained in light has been lost in definition. At times—perhaps on one occasion in fifty—this experience is different; the atmosphere has apparently assumed a state of quiescence, and objects are seen in a great telescope with the same clearness of detail as in smaller ones. It is then the observer fully realizes that his instrument, though generally ineffective, is not itself in fault, and that it would do valuable work were the normal condition of the air suitable to the exercise of its capacity.

Those who have effected discoveries with large instruments have done so in spite of the impediment alluded to. Relying mainly upon great illuminating power, bad or indifferent definition has been tolerated; and they have succeeded in detecting minute satellites, faint nebulæ, clusters, and small companions to double stars. Telescopes of great aperture are at home in this kind of work. But when we come to consider discoveries on the surfaces of the Sun, Moon, and planets, the case is entirely different; the diligent use of small appliances appears to have left little for the larger constructions to do. There are some thousands of drawings of the objects named, made by observers employing telescopes from 3 up to 72 inches in diameter; and a careful inspection shows that the smaller instruments have not been outdone in this interesting field of observation. In point of fact they rather appear to have had the advantage, and the reason of this is perhaps sufficiently palpable. The details on a bright planetary object are apt to become obliterated in the glare of a large instrument. Even with a small telescope objects like Venus and Jupiter are best seen at about the time of sunset, and before their excessive brilliancy on the dark sky is enabled to act prejudicially in effacing the delicate markings. Probably this is one of the causes which, in combination with the undulations of the atmosphere, have restricted the discoveries of large instruments chiefly to faint satellites, stars, and nebulæ.

Prof. Young ascribes many of the successes of small instruments to exceptional cuteness of vision on the part of certain observers, and to the fact that such instruments are so very numerous and so diligently used that it is fair to conclude they must reap the main harvest of discoveries. We must remember that for every observer working with an aperture of 18 inches and more, there are more than a hundred employing objectives or specula of from 5 to 12 inches; hence we may expect some notable instances of keen sight amongst the latter. The success of men like Dawes and others, who outstrip their contemporaries, and with small glasses achieve phenomenal results, is to be ascribed partly to good vision and partly to that natural aptitude and pertinacity uniformly characteristic of the best observers. These circumstances go far to explain the unproductiveness of large telescopes: in the race for distinction they are often distanced by their more numerous and agile competitors.

The objections which applied to the large reflecting instruments of Herschel, Lassell, and Rosse scarcely operate with the same force in regard to the great refractors of the present day, and for these reasons:—Refractors are somewhat less sensitive to atmospheric disturbances than reflectors. The modern instruments are mounted in much improved style, and placed in localities selected for their reception. In fact, all that the optician’s art can do to perfect such appliances has been done, and Nature herself has been consulted as to essentials; for we find the most powerful refractor of all erected on the summit of Mount Hamilton, where the skies are clear and Urania ever smiles invitingly.

Some observers who have obtained experience both with large and small telescopes aver that, even on a bright planet, they can see more, and often see it much better, with the larger glasses. But we rarely, if ever, find them saying they can discern anything which is absolutely beyond the reach of small instruments. It would be much more satisfactory evidence of the super-excellence of the former if definite features could be detected which are quite beyond the reach of telescopes of inferior size; but we seldom meet with experiences of this kind, and the inference is obvious.

There is undoubtedly a certain aperture which combines in itself sufficient light-grasping power with excellent definition. It takes a position midway between great illuminating power and bad definition on the one hand, and feeble illuminating power and sharp definition on the other. Such an aperture must form the best working instrument in an average situation upon ordinary nights and ordinary objects. M. Wolf fixes this aperture at about 15 inches, and he is probably near the truth.

The quaint Dr. Kitchener, who, early in the present century, made a number of trials with fifty-one telescopes, entertained a very poor opinion of big instruments. In his book on ‘Telescopes,’ he says:—“Immense telescopes are only about as useful as the enormous spectacles suspended over the doors of opticians.” ... “Astronomical amateurs should rather seek for perfect instruments than large ones. What good can a great deal of bad light do?”

We shall be in a better position a few years hence to estimate the value of great telescopes; for the principal instruments of this class have only been completed a short time. Judging from the statements of some of the observers, who are men of the utmost probity and ability, certain of the large instruments are capable of work far in advance of anything hitherto done. Definition, they say, is excellent, notwithstanding the great increase of aperture. The old stumbling-block appears, therefore, to have been removed, and astronomy is to be congratulated on the acquirement of such vastly improved implements of research. Even should the large telescopes continue to prove disappointing in certain branches, they may certainly be expected to maintain their advantage in others. They will always be valuable as a corrective to smaller and handier instruments. For special lines of work in which very small or very faint objects are concerned, considerable light-grasping power is absolutely required; and it is chiefly in these departments that large instruments may be further expected to augment our knowledge. In photographic and spectroscopic work they also have a special value, which late researches have brought prominently to the fore.

The telescopes of the future will probably surpass in dimensions those of our own day. The University of Los Angelos, in California, propose to erect a 42-inch refractor on the summit of Wilson’s Peak of the Sierra Madre mountains, which is 6000 feet high and about 25 miles from Los Angelos. In reference to this contemplated extension of size, it may be opportune to mention that large objectives do not transmit light proportionately with their increased diameter, owing to greater thickness of the lenses, which increases the absorption. The Washington objective of 25·8-inch aperture is 2·87 inches in thickness, and more than half the light which falls upon it is lost by absorption. On the other hand, specula, with every enlargement of aperture, give proportionately more light-grasping power, and their diameters might be greatly increased but for the mechanical obstacles in the way of their construction. Mr. Ranyard expresses the opinion that “with the refractor we are fast approaching the practical limit of size.” After referring to the Washington object-glass as above, he says:—“If we double the thickness, more than three quarters of the light would be absorbed and less than one quarter would be transmitted. The greatest loss of light is only for the centre of the object-glass; but in all parts the absorption is quadrupled for a lens of double aperture.” If, therefore, future years see any great development in the sizes of telescopes, it will probably be in connection with reflectors; for the loss of light by absorption in the thick lenses of large refractors must ultimately determine their limits. Mr. Calver says:—“The light of reflectors exceeding 18 inches in diameter is certainly greater than that of refractors of equal size, and for anything like 3 feet very much greater.” He nearly obtained the order for a monster reflector for the Lick Observatory, the Americans admitting that the reflector must be the instrument of the future for power and light because there were practically no limits to its size. But the reflector has not been much used in America, and therefore is little known. For this reason the authorities decided to erect a large refractor, and they appear to have been justified in their selection, for the 36-inch objective has proved excellent.

[CHAPTER III.]
NOTES ON TELESCOPES AND THEIR ACCESSORIES.

Choice of Telescopes.—Refractors and Reflectors.—Observer’s Aims.—Testing Telescopes.—Mounting.—Eyepieces.—Requisite Powers.—Overstating Powers.—Method of finding the Power.—Field of Eyepiece.—Limited Means no obstacle.—Observing-Seats.-Advantage of Equatoreals.—Test-Objects.—Cheapness and increasing number of Telescopes.—Utility of Stops.—Cleaning Lenses.—Opera-Glasses.—Dewing of Mirrors.—Celestial Globe.—Observatories.

Choice of Telescopes.—The subject of the choice of telescopes has exercised every astronomer more or less, and the question as to the best form of instrument is one which has occasioned endless controversy. The decision is an important one to amateurs, who at the outset of their observing careers require the most efficient instruments obtainable at reasonable cost. It is useless applying to scientific friends who, influenced by different tastes, will give an amount of contradictory advice that will be very perplexing. Some invariably recommend a small refractor and unjustly disparage reflectors, as not only unfitted for very delicate work, but as constantly needing re-adjustment and resilvering[6].

Others will advise a moderate-sized reflector as affording wonderfully fine views of the Moon and planets. The question of cost is greatly in favour of the latter construction, and, all things considered, it may claim an unquestionable advantage. A man who has decided to spend a small sum for the purpose not merely of gratifying his curiosity but of doing really serviceable work, must adopt the reflector, because refractors of, say, 5 inches and upwards are far too costly, and become enormously expensive as the diameter increases. This is not the case with reflectors; they come within the reach of all, and may indeed be constructed by the observer himself with a little patience and ingenuity.

Refractors and Reflectors.—The relative merits of refractors and reflectors[7] have been so frequently compared and discussed that we have no desire to re-open the question here. These comparisons have been rarely free from bias, or sufficiently complete to afford really conclusive evidence either way. There is no doubt that each form of instrument possesses its special advantages: aperture for aperture the refractor is acknowledged to be superior in light-grasping power, but the ratio given by different observers is not quite concordant. A silver-on-glass mirror of 8-inches aperture is certainly equal to a 7-inch objective in this respect, while as regards dividing power and the definition of planetary markings, the reflector is equal to a refractor of the same aperture. The much shorter focal length of the reflector is an advantage not to be overlooked. A century ago Sir W. Herschel figured his specula to foci of more than a foot to every inch of aperture, except in the case of his largest instruments. Thus he made specula of 18½-inches and 24-inches diameter, the former of which had a focal length of 20 feet and the latter of 25 feet. The glass mirrors of the present time are much shorter, and the change has not proved incompatible with excellent performance. Calver has made two good mirrors of 17-1/4-inches aperture, and only 8 ft. 4 in. focus. Mr. Common’s 5-foot mirror is only 27½ feet, so that in these instances the length of the tube is less than six times the diameter.

Fig. 12.

“The Popular Reflector” by Calver.

It has long been proved that refractors and reflectors alike are, in good hands, capable of producing equally good results; and we may depend upon it that, in spite of all argument and experiment, both kinds of telescope will continue to hold their own until superseded by a new combination, which hardly seems likely. If the observer is free from prejudice, he will have no cause to deplore the character of his instrument, always supposing it to be by a good maker. Be it object-glass or speculum, he will rarely find it lacking in effectiveness. It happens only too often that the telescope or the atmosphere is hastily blamed when the fault rests with the observer himself. Let him be persistent in waiting opportunities, and let the instrument be nicely adjusted and in good condition, and in the great majority of cases it will perform all that can reasonably be expected of it.

In choosing appliances for observational purposes, the observer will of course be guided by his means and requirements. If his inclination lead him to enter a particular department of research, he will take care to provide himself with such instruments as are specially applicable to the work in hand. Modern opticians have effected so many improvements, and brought out so many special aids to smooth the way of an observer, that it matters little in which direction he advances; he will scarcely find his progress impeded by want of suitable apparatus. In size, as also in character, the observer should be careful to discriminate as to what is really essential. Large instruments and high powers are not necessary to show what can be sufficiently well seen in a small telescope with moderate power. Of course there is nothing like experience in such matters, and practice soon renders one more or less proficient in applying the best available means.

Fig. 13.

3-inch Refracting-Telescope, by Newton & Co.

An amateur who really wants a competent instrument and has to consider cost, will do well to purchase a Newtonian reflector. A 4½-inch refractor will cost about as much as a 10-inch reflector, but, as a working tool, the latter will possess a great advantage. A small refractor, if a good one, will do wonders, and is a very handy appliance, but it will not have sufficient grasp of light for it to be thoroughly serviceable on faint objects. Anyone who is hesitating in his choice should look at the cluster about χ Persei through instruments such as alluded to, and he will be astonished at the vast difference in favour of the reflector. For viewing sun-spots and certain lunar objects small refractors are very effective, and star-images are usually better seen than in reflectors, but the latter are much preferable for general work on account of three important advantages, viz., cheapness, illuminating power, and convenience of observation. When high magnifiers are employed on a refractor of small aperture, the images of planets become very faint and dusky, so that details are lost.

Observer’s Aims.—If the intending observer merely requires a telescope to exhibit glimpses of the wonders which he has seen portrayed in books, and has no intention of pursuing the subject further than as an occasional hobby, he will do well to purchase a small refractor between 3 and 4 inches in aperture. Such instruments are extremely effective on the Sun and Moon, which are naturally the chief objects to attract attention, and, apart from this, appliances of the size alluded to may be conveniently used from an open window. The latter is an important consideration to many persons; moreover, a small telescope of this kind will reveal an astonishing number of interesting objects in connection with the planets, comets, &c., and it may be employed by way of diversion upon terrestrial landscape, as such instruments are almost invariably provided with non-inverting eyepieces. Out-of-door observing is inconvenient in many respects, and those who procure a telescope merely to find a little recreation will soon acknowledge a small refractor to be eminently adapted to their purposes and conveniences.

Those who meditate going farther afield, and taking up observation habitually as a means of acquiring practical knowledge, and possibly of doing original work, will essentially need different means. They will require reflectors of about 8 or 10 inches aperture; and, if mounted in the open on solid ground, so much the better, as there will be a more expansive view, and a freedom from heated currents, which renders an apartment unsuited to observations, unless with small apertures where the effects are scarcely appreciable. A reflector of the diameter mentioned will command sufficient grasp to exhibit the more delicate features of planetary markings, and will show many other difficult objects in which the sky abounds. If the observer be specially interested in the surface configuration of Mars and Jupiter he will find a reflector a remarkably efficient instrument. On the Moon and planets it is admitted that its performance is, if not superior, equal to that of refractors. If, however, the inclination of the observer leads him in the direction of double stars, their discovery and measurement, he will perhaps find a refractor more to be depended upon, though there is no reason why a well-mounted reflector should not be successfully employed in this branch; and the cost of a refractor of the size to be really useful as an instrument of discovery must be something very considerable—perhaps ten times as great as that of a reflector of equal capacity. As far as my own experience goes the refractor gives decidedly the best image of a star. In the reflector, a bright star under moderately high power is seen with rays extending right across the field, and these appear to be caused by the supports of the flat.

Testing Telescopes.—No amateur should buy an instrument, especially a second-hand one, without testing it, and this is a delicate process involving many points to be duly weighed. Experience is of great service in such matters, and is, in fact, absolutely necessary. Even old observers are sometimes misled as to the real worth of a glass. In such cases, there is nothing like having a reliable means of comparison, i. e. another telescope of acknowledged excellence with which to test the doubtful instrument. In the absence of such a standard judgment will be more difficult, but with care a satisfactory decision may be arrived at. The Moon is too easy an object for the purpose of such trials; the observer should rather select Venus or Jupiter. The former is, however, so brilliant on a dark sky, and so much affected with glare, that the image will almost sure to be faulty even if the glass is a good one. Let the hour be either near sunrise or sunset, and if the planet has a tolerably high altitude, her disk ought to be seen beautifully sharp and white. Various powers should be tried, increasing them each time, and it should be noticed particularly whether the greater expansion of the image ruins the definition or simply enfeebles the light. In a thoroughly good glass faintness will come on without seriously impairing the definite contour of the object viewed, and the observer will realize that the indistinctness is merely occasioned by the power being relatively in excess of the light-grasp. But in a defective telescope, a press of magnifying power at once brings out a mistiness and confuses the details of the image in a very palpable manner. Try how he will, the observer will find it impossible to get rid of this, except, perhaps, by a “stop” which cuts off so much light that the instrument is ineffective for the work required of it. The blurred image is thought, at the moment of its first perception, to be caused by the object being out of focus, and the observer vainly endeavours to get a sharper image until he finds the source of error lies elsewhere. A well-figured glass ought to come very sharply to a focus. The slightest turn of the adjusting-screw should make a sensible difference. On the other hand, an inferior lens will permit a slight alteration of focusing without affecting the distinctness, because the rays from the image are not accurately thrown to a point. Jupiter is also a good test. The limbs of the planet, if shown clean and hard, and the belts, if they are pictured like the finely cut details of an engraving, will at once stamp a telescope as one of superior quality. Saturn can also be examined though not, perhaps, so severe a test. The belts, crape ring, Cassini’s division, ought to be revealed in any telescope of moderate aperture. If, with regard to any of these objects, the details apparently run into each other and there is a “fuzzy” or woolly aspect about them which cannot be eliminated by careful focusing, then either the atmosphere or the telescope is in fault. If the former, another opportunity must be awaited. An observer of experience will see at a glance whether the cause lies in the air or the instrument. The images will be agitated by obnoxious currents, if the defects are due to the atmosphere, but if the glass itself is in error, then the objects will be comparatively tranquil but merged in hazy outlines, and a general lack of distinctness will be apparent. Perhaps the best test of all as to the efficiency of a telescope is that of a moderately bright star, say of the 2nd or 3rd magnitude. With a high power the image should be very small, circular, and surrounded by two or three rings of light lying perfectly concentric with each other. No rays, wings, or extraneous appearance other than the diffraction rings should appear.

This, however, specially applies to refractors, for in reflectors the arms of the flat occasion rays from any bright star; I have also seen them from Mars, but of course this does not indicate an imperfect mirror. If there is any distortion on one side of the image, then the lenses are inaccurately centred though the instrument may be otherwise good, and a little attention may soon set matters right. When testing a glass the observer should choose objects at fairly high altitudes, and not condemn a telescope from a single night’s work unless the evidence is of unusually convincing character. If false colour is seen in a silver-on-glass reflector it is originated by the eyepiece, though not necessarily so in a refractor. The object-glass of the latter will be sure to show some uncorrected colour fringing a bright object. A good lens, when exactly focused, exhibits a claret tint, but within the focus purple is seen and beyond the focus green comes out. In certain cases the secondary spectrum of an object-glass is so inadequately corrected that the vivid colouring of the images is sometimes attributed by inexperienced observers to a real effect. A friend who used a 3-inch refractor once called on me to have a glimpse of Jupiter through my 10-inch With-reflector. On looking at the planet he at once exclaimed “But where are the beautiful colours, Mr. Denning?” I replied to his question by asking another, viz., “What colours?” he answered, “Why, the bright colours I see round Jupiter in my refractor?” I said, “Oh, they exist in your telescope only!” He looked incredulous, and when he left me that night did not seem altogether pleased with the appearance of Jupiter shorn of his false hues!

Mounting.—Too much care cannot be given to the mounting of telescopes, for the most perfectly figured glass will be rendered useless by an inefficient stand; a faulty lens, if thoroughly well mounted, will do more than a really good one on a shaky or unmanageable mounting. Whatever form is adopted, the arrangement should ensure the utmost steadiness, combined with every facility for readily following objects. A man who has every now and then to undergo a great physical exertion in bodily shifting the instrument is rendered unfit for delicate work. The telescope should be provided with every requisite for carrying on prolonged work with slight exertion on the part of the observer. Unless the stand is firm there will be persistent vibrations, especially if the instrument is erected in the open, for there are very few nights in the year when the air is quite calm. These contingencies should be provided against with scrupulous attention if the observer would render his telescope most effective for the display of its powers, and avoid the constant annoyance that must otherwise follow.

Fig. 14.

Huygens’s negative eyepiece.

Fig. 15.

Ramsden’s positive eyepiece.

Eyepieces.—Good eyepieces are absolutely essential. Many object-glasses and specula have been deprecated for errors really originated by the eyepiece. Again, telescopes have not unfrequently been blamed for failures through want of discrimination in applying suitable powers. A consistent adaptation of powers according to the aperture of the telescope, the character of the object, the nature of the observation, and the atmospheric conditions prevailing at the time, is necessary to ensure the best results. If it is required to exhibit a general view of Jupiter and his satellites to a friend, we must utilize a low power with a large field; if, on the other hand, we desire to show the red spot and its configuration in detail, we must apply the highest power that is satisfactorily available. The negative or Huygenian eyepiece is the one commonly used, and it forms good colourless images, though the field is rather small. The positive or Ramsden eyepiece gives a flatter and larger field, but it is not often achromatic. A Kellner eyepiece, the feature of which is a very large field, is often serviceable in observations of nebulæ, clusters, and comets. Telescopes are sometimes stated to bear 100 to the inch on planets, but this is far beyond their capacities even in the very best condition of air. Amateurs soon find from experience that it is best to employ those powers which afford the clearest and most comprehensive views of the particular objects under scrutiny. Of course when abnormally high powers are mentioned in connection with an observation, they have an impressive sound, but this is all, for they are practically useless for ordinary work. I find that 40, or at the utmost 50 to the inch, is ample, and generally beyond the capacities of my 10-inch reflector. A Barlow lens used in front of the eyepiece raises the power about one third, and thus a whole set of eyepieces may be increased by its insertion. It is said to improve the definition, while the loss of light is very trifling. I formerly used a Barlow lens in all planetary observations, but finally dispensed with it, as I concluded the improved distinctness did not compensate for the fainter image. A great advantage, both in light and definition, results from the employment of a single lens as eyepiece. True, the field is very limited, and, owing to the spherical aberration, the object so greatly distorted near the edges that it must be kept near the centre, but, on the whole, the superiority is most evident. By many careful trials I find it possible to glimpse far more detail in planetary markings than with the ordinary eyepiece. Dawes, and other able observers, also found a great advantage in the single lens, and Sir W. Herschel, more than a century ago, expressed himself thus:—“I have tried both the double and single lens eye-glass of equal powers, and always found that the single eye-glass had much the superiority in light and distinctness.”

Requisite Powers.—For general purposes I believe three eyepieces are all that is absolutely requisite, viz., a low power with large field for sweeping up nebulæ and comets; a moderate power for viewing the Moon and planets; and a high power for double stars and the more delicate forms on the planets. For a 3-inch refractor, eyepieces of about 15, 75, and 150 would be best, and for a 10-inch reflector 40, 150, and 300. For very difficult double stars a still higher power will be occasionally useful, say 250 for the refractor, and 500 for the reflector. The definition usually suffers so much under high powers, and the tremors of the atmosphere are brought out so conspicuously, that the greater expansion of the image of a planet does not necessarily enable it to present more observable detail. The features appear diluted and merged in hazy outlines, and there is a lack of the bright, sharply determinate forms so steadily recognized under lower magnifiers. In special cases great power may become essential, and, under certain favourable circumstances, will prove really serviceable, but, in a general way, it is admitted that the lowest power which shows an object well is always the best. I have occasionally obtained very fair views of Saturn with a power of 865, but find that I can perceive more of the detail with 252. Some daylight observations of Venus were also effected under very high power, and, though the definition remained tolerably good, I found as the result of careful comparison that less power answered more satisfactorily. But it would be absurd to lay down inviolable rules in such cases. Special instruments, objects, and circumstances require special powers, and observers may always determine with a little care and experience the most eligible means to support their endeavours. One thing should be particularly remembered, that the power used must not be beyond the illuminating capacity of the instrument, for planetary features appear so faint and shady under excessive magnifiers that nothing is gained. To grasp details there must be a fair amount of light. I have seen more with 252 on my 10-inch reflector than with 350 on a 5-1/4-inch refractor, because of the advantage from the brighter image in the former case.

Overstating Powers.—It seems to be a fashionable imposition on the part of opticians to overstate magnifying powers. Eyepieces are usually advertised at double their true strength. My own 10-inch reflector was catalogued as having four eyepieces, 100 to 600, but on trial I found the highest was no more than 330. This custom of exaggerating powers seems to have long been a privileged deception, and persons buying telescopes ought to be guarded against it. Dr. Kitchiner says it originated with the celebrated maker of reflectors, James Short, and justly condemns it as a practice which should be discontinued. I suppose it is thought that high powers advertised in connection with a telescope have an exalted sound and are calculated to attract the unwary purchaser; but good instruments need no insidious trade artifices to make them saleable. The practice does not affect observers of experience, because it is well understood, and they take good care to test their eyepieces directly they get them. But the case is different with young and inexperienced amateurs, who naturally enough accept the words of respectable opticians, only to find, in many cases, that they have been misleading and a source of considerable annoyance.

Method of finding the Power.—The magnifying power of a telescope may be determined by dividing the focal length of the object-glass or mirror by the focal length of the eye-lens. Thus, if the large glass has a focus of 70 inches and the eye-lens a focus of one inch, then the power is 70. If the latter is only 1/4-inch focus, the resulting power will be 280. But this method is only applicable to single lens eyepieces. We may, however, resort to several other means of finding the powers of the compound eyepieces of Huygens or Ramsden. Let the observer fix a slip of white cardboard, say 1 inch wide, to a door or post some distance off, and then (with a refractor) view it, while keeping the disengaged eye open, and note the exact space covered by the telescopic image of the card as projected on the door seen by the other eye. The number of inches included in the space alluded to will represent the linear magnifying power. A brick wall or any surface with distinct, regularly marked divisions will answer the same purpose, the number of bricks or divisions covered by the telescopic image of one of them being equivalent to the power. But it should not be forgotten that a telescope magnifies slightly less upon a celestial object than upon a near terrestrial one owing to the shorter focus, and a trifling allowance will have to be made for this. Another plan may be mentioned. When the telescope is directed to any fairly bright object or to the sky, and the observer removes his eye about 10 inches from the eyepiece, a sharply defined, bright little disk will be perceived in the eye-lens. If the diameter of this disk is ascertained and the clear aperture of the object-glass or mirror is divided by it, the quotient will be the magnifying power. Thus, if the small circle of light is ·2 inch diameter and the effective aperture of the large glass 5 inches, then the power is 25. If the former is ·02 inch diameter and the latter 7·5 inches, the power will be 375. The dynamometer is a little instrument specially designed to facilitate this means of fixing the magnifying power. It enables the diameter of the small luminous circle in the eye-lens to be very accurately measured, and this is a most important factor in deriving the power by this method.

Fig. 16.

Berthon’s Dynamometer. Horne & Thornthwaite London.

Field of Eyepiece.—Observers often require to know the diameter of the fields of their eyepieces. Those engaged in sweeping up comets, nebulæ, or other objects requiring large fields and low powers, find it quite important to have this information. They may acquire it for themselves by simple methods. A planet, or star such as δ Orionis, η or γ Virginis, or η Aquilæ, close to the equator, should be allowed to run exactly through the centre of the field, and the interval occupied in its complete transit from ingress to egress noted several times. The mean result in min. and sec. of time must then be multiplied by 15, and this will represent the diameter required in min. and sec. of arc on the equator. A planet or star near the meridian is the best for the purpose. If the object occupies 1 min. 27 sec. of time in passing from the E. to the W. limit of the field, then 87 sec. × 15 = 1305″, or 21′ 45″. A more accurate method of deriving the angle subtended by the field is to let a star, say Regulus, pass through the centre, and fix the time which lapses in its entire passage by a sidereal clock; then the interval so found × 15 × cosine of the declination of Regulus will indicate the diameter of the field. Suppose for instance, that the star named occupies 2 min. 14 sec. = 134 sec. in its passage right across the whole and central part of the field: then

so that the diameter of the field of the eyepiece must be 32′ 42″, nearly corresponding with the diameter of the Moon.

Limited Means no Obstacle.—There are many observers who, having limited means, are apt to consider themselves practically unable to effect good work. This is a great illusion. There are several branches of astronomy in which the diligent use of a small instrument may be turned to excellent account. Perseverance will often compensate for lack of powerful appliances. Many of the large and expensive telescopes, now becoming so common, are engaged in work which could be as well performed with smaller aperture, and when the manifold advantages of moderate instruments are considered, amateurs may well cease to deplore the apparent insufficiency of their apparatus. It is, however, true that refractors have now attained dimensions and a degree of proficiency never contemplated in former times, and that the modern ingenuity of art has given birth to innumerable devices to facilitate the work of those engaged in observation. In many of our best appointed observatories the arrangements are so very replete with conveniences, and so sedatory in their influences, that the observer has every inducement to fall asleep, though we do not find instances of “nodding” recorded in their annals. Further progress in the same direction leads us to joyfully anticipate the time when, instead of standing out in the frost, we may comfortably make our observations in bed. This will admirably suit all those who, like Bristol people, are reported to sleep with one eye open! But, to be more serious, the work of amateurs is much hindered by lack of means to construct observatories wherein they may conduct researches without suffering from all the rigours of an unfavourable climate. Many of them have, like William Herschel a century ago, to pursue their labours under no canopy but the heavens above, and are exposed to all the trying severity of frost and keen winds, which keep them shivering for hours together, and very much awake!

Fig. 17.

Cooke and Son’s Educational Telescope.

Observing-Seats.—As to observing-seats, many useful contrivances have been described from time to time in the ‘Astronomical Register’ and ‘English Mechanic.’ Some of these answer their design admirably, but I believe a good chair, embodying all the many little requirements of the observer, yet awaits construction. Those I have seen, while supplying certain acknowledged wants, are yet deficient in some points which need provision. With my reflector I find an ordinary step-ladder answers the purpose very well. It is at once light, simple, and durable, and enables observations to be secured at any altitude. It may be readily placed so that the observer can work in a sitting posture, and the upper shelves, while convenient to lean upon, may be so arranged as to hold eyepieces, and are to be further utilized when making drawings at the telescope. I find it possible to obtain very steady views of celestial objects in this way. Everyone knows that during a critical observation it is as essential for the observer to be perfectly still as it is for the instrument to be free from vibration. A person who stands looking through a telescope feels a desire to ensure a convenient stability by catching hold of it. The impression is no doubt correctly conveyed to his mind that he may obtain a better view in this way; and so he would, were it not for the dancing of the image which instantly follows the handling of the instrument. For this reason it is absolutely necessary that no part of the observer touch the telescope while in use. He must ensure the desired steadiness, which is really a most important consideration, by other means; and an observer who provides for this contingency will have taken a useful step in the way of achieving delicate work.

Advantage of Equatoreals.—Those who employ equatoreal mounting and clock-work will manifestly command an advantage in tracing features on a planet or other object requiring critical scrutiny. Common stands, though often good make-shifts, require constant application on the part of the observer, when his undivided attention should be concentrated on the object. With an alt-azimuth stand nearly one half the observer’s time is occupied in keeping the object near the middle of the field. Though good views are obtainable, they are very fugitive. Just as the delicate features are being impressed on the retina they are lost in the ill-defined margin of the field, or from the necessity of suddenly shifting the object back. A succession of hurried views of this kind, during which the observer is frantically endeavouring to grasp details which only require a steady view to be well displayed, are often tantalizing and seldom satisfactory in their issues. This is especially the case when a single lens and high powers are used, and if the night is windy the difficulty is intensified. It is, therefore, evident that a clock-driven telescope possesses marked advantages in delicate work on faint objects, because the prolonged view better enables the eye to gather in the details which are all but lost in the elusive glimpses afforded by inferior means. Still we must not forget that rough appliances do not present an effectual barrier to success. The very finest definition comes only in momentary glimpses. The sharply-cut outlines of planetary configuration cannot steadily be held for long together. Only now and then the image acquires the distinctness of an engraving, when the air and the focus of eye and telescope severally combine to produce a perfect picture. Observers, therefore, whose instruments are simply, though perhaps substantially mounted in handy fashion, must profit by these moments of fine seeing, and, when drawing, will find it expedient to fill in, little by little, the delicate forms which reach the eye. This will take much time owing to the drawbacks alluded to, but the outcome will more than justify its expenditure, and the observer will gain patience and perseverance which will prove a useful experience in the future.

Lenses out of centre or misplaced are, like other defects, calculated to give rise to errors as numerous as they are various. But the most striking of these apparently belong to a period when telescopes were far less perfect and popular than at the present day. Indeed, it is surprising that so very few false or imaginary discoveries are announced when we consider the vast array of instruments that are now employed. It is true we occasionally hear that a comet has been discovered close to Jupiter, that several companions have been seen to Polaris, or that some other extraordinary “find” has been effected, but the age is dead when such announcements were accepted without suitable investigation. The satellite of Venus has long since ceased to exist. The active volcanoes on the Moon have become extinct. Even Vulcan will have to be set aside, and, like many another sensation which caused quite a furóre in its day, must soon be altogether expunged from the category of “suspects.”

Test-objects. Opticians sometimes advertise lists of objects—generally double stars—which may be seen with their instruments, but it does not appear to be sufficiently understood that the character of a telescope is dependent in a great degree upon the ability of the observer, who can either make or mar it, according to the skill he displays in its management. Some men will undoubtedly see more with 5 inches of aperture than others will with 10. Certain observers appear to excel in detecting delicate planetary markings, while others possess special aptitude for glimpsing minute objects such as faint satellites, or comites to double stars, and the explanation seems to be that partly by experience and partly by differences in the sensitiveness of vision, exceptional powers are sometimes acquired in each of these departments. The various test-objects which have been given by reliable authorities, though representing average attainments, are not applicable to the abnormal powers of vision possessed by certain observers. In fact, the capacity of a telescope cannot be correctly assigned and its powers circumscribed by arbitrary rules, because, as already stated, the character of the observer himself becomes a most important factor in this relation. Climatic influences have also considerable weight, though less so than the personal variations referred to, for one man will succeed, where another meets with utter failure. This is unquestionably due to differences in eyesight, method, and experience. But whatever the primary causes may be, everyone knows they induce widely discordant results, and occasion many of the contradictions which become the subjects of controversy. And, as a rule, amateurs should avoid controversy, because it very rarely clears up a contested point. There is argument and reiteration, but no mutual understanding or settlement of the question at issue. It wastes time, and often destroys that good feeling which should subsist amongst astronomers of every class and nationality. In cases where an important principle is involved, and discussion promises to throw light upon it, the circumstances are quite different. But paltry quibblings, fault-finding, or the constant expression of negative views, peculiar to sceptics, should be abandoned, as hindering rather than accelerating the progress of science. Let observers continually exercise care and discretion and satisfy themselves in every legitimate way as to the accuracy of their results, and they may fearlessly give them expression and overcome any objections made to their acceptance. They should accord one another an equal desire for the promotion of truth. Competition and rivalry in good spirit increase enthusiasm, but there is little occasion for the bitterness and spleen sometimes exhibited in scientific journals. There are some men whose reputations do not rest upon good or original work performed by themselves, but rather upon the alacrity with which they discover grievances and upon the care they will bestow in exposing trifling errors in the writings of their not-infallible contemporaries. Such critics would earn a more honourable title to regard were they to devote their time to some better method of serving the cause of science.

Cheapness and increasing number of Telescopes.—A marked feature of optical instruments is their increasing cheapness. Little more than half a century ago Tulley charged £315 for a 10-inch Newtonian reflector. At the present time Calver asks £50 for an instrument of the same aperture, and sometimes one may be picked up, second-hand, for half of that amount. Not only have telescopes become cheaper, but they have greatly improved in performance since silvered glass superseded the metallic speculum. Hence we find moderately-powerful instruments in the hands of a very large number of observers. Astronomical publications have proportionately increased, so that amateurs of to-day can boast of facilities, both of making and recording observations, which were scarcely dreamt of a century ago. It must be admitted, however, that the results hardly do justice to the means available. Such an enormous number of telescopes are variously employed that one cannot avoid a feeling of surprise at the comparative rarity of new discoveries, and, indeed, of published observations generally. It is certain that the majority of existing telescopes are either lying idle or applied in such a desultory fashion as to virtually negative the value of the results. Others, again, are indiscriminately employed upon every diversity of object without special aim or method, and with a mere desire to satisfy curiosity. Now it is to be greatly deplored that so much observing strength is either latent or misdirected. The circumstances obviously demand that an earnest effort should be made to utilize and attract it into suitable channels. To do this effectually, the value of collective effort should be forcibly explained, the interest and enthusiasm of observers must be aroused in a permanent manner, and they must be banded together according to their choice of subjects. An effort in this direction has been made by the Liverpool Astronomical Society, and the results have proved distinctly favourable; a considerable amount of useful work has been effected in several branches and it forms the subject of some valuable reports which have been annually published in the ‘Journal.’

Utility of Stops.—There are a good many details connected with observation which, though advice may be tendered in a general way, are best left to the discrimination of observers, who will very soon discover their influences by practical trial and treat them accordingly. The employment of stops or diaphragms to contract the aperture of telescopes is a question on which a diversity of opinion has been expressed. It is often found, on nights of indifferent seeing, that the whole aperture, especially of a faulty instrument, gives bad images, and that, by reducing it, definition becomes immensely improved. But Mr. Burnham, the double star observer, records his opinion that a good glass needs no contraction, and that the whole aperture shows more than a part unless there is defective figuring at the outer zone of the lens, which will be cut off by the stop and its performance thereby greatly improved. He seems to think that a glass requiring contraction is essentially defective, but this is totally opposed to the conclusions of other observers. It is almost universally admitted that, on bad nights, the advantages of a large aperture are neutralized by unsteady definition, and that, by reducing the diameter, the character of the images is enhanced. As regards instruments of moderate calibre the necessity is less urgent. With my 10-inch reflector I rarely, if ever, employ stops, for by reducing the aperture to 8 inches the gain in definition does not sufficiently repay for the serious loss of light. But in the case of large telescopes the conservation of light is not so important, and a 14-inch or 16-inch stop may be frequently employed on an 18-inch glass with striking advantage. The theory that only defective lenses improve with contraction is fallacious, for in certain cases where stops are regularly employed it is found that, under circumstances of really good seeing, the whole aperture gives images which are as nearly perfect as possible. It is clear from this that the fault lies with the atmosphere, and that under bad conditions it becomes imperative to limit its interference consistently with the retention of sufficient light to distinguish the object well. In large reflectors, particularly, the undulations of the air are very active in destroying definition, and the fact will be patent enough to anyone who compares the images given in widely different apertures. The hard, cleanly cut disks shown by a small speculum or object-glass offer an attractive contrast to the flaring, indefinite forms often seen in big telescopes.

Cleaning Lenses.—As to wiping objectives or mirrors, this should be performed not more often than absolute necessity requires; and in any case the touches should be delicate and made with materials of very soft texture. The owner of a good objective should never take the handkerchief out of his pocket and, in order to remove a little dust or dew, rub the glass until the offensive deposit is thought to be removed. Yet this is sometimes done, though frequent repetition of such a process must ultimately ruin the best telescope notwithstanding the hardness of the crown glass forming the outer lens of the objective. It will not bear such “rough and ready” usage and in time must show some ugly scratches which will greatly affect its value though they may not seriously detract from its practical utility. Good tools deserve better treatment. When the glass really wants cleaning, remove it from the tube and sweep its whole surface gently with a dry camel’s-hair brush, or when this is not at hand get a piece of linen and “flick” off the dust particles. Then wipe the lens, as soon as these have been dislodged, with an old silk, or soft cambric handkerchief; fine chamois leather is also a good material, and soft tissue paper, aided by the breath, has been recommended. But whatever substance may be adopted it must be perfectly clean and free from dust. When not in use it should be corked up in a wide-necked bottle where it will be safe from contact with foreign particles. In the case of mirrors there is an obvious need that, when being repolished, the material used should be perfectly dry and that the mirror also should be in the same state. It is unnecessary to say here that in no case must the silver film be touched when it is clouded over with moisture. This must first be allowed to evaporate in a free current of air or before a fire; the former is to be preferred. A suitable polishing-pad may be made with a square piece of washleather or chamois in which cotton-wool is placed and then tied into a bag. This may be dipped into a little of the finest rouge, and its employment will often restore a bright surface to the mirror. But the latter should be left “severely alone” unless there is urgent occasion to repolish it, as every application of the rouged pad wears the film and may take off minute parts of it, especially when dust has not been altogether excluded. The precarious nature of the silvered surface undoubtedly constitutes the greatest disadvantage of modern reflectors. The polish on the old metallic mirrors was far more durable. Some of Short’s, figured 150 years ago, still exist and are apparently as bright as when they were turned out of the workshop! I have a 4-inch Gregorian by Watson which must be quite a century old, and both large and small specula seem to have retained their pristine condition.

With regard to the duration of the silver-on-glass films, much of course depends upon the care and means taken to preserve them. Calver says that sometimes the deposit does not last so long as expected, though he has known the same films in use for ten years. A mirror that looks badly tarnished and fit for nothing will often perform wonderfully well. With my 10-inch in a sadly deteriorated state I have obtained views of the Moon, Venus, and Jupiter that could hardly be surpassed. The moderate reflection from a tarnished mirror evidently improves the image of a bright object by eliminating the glare and allowing the fainter details to be readily seen. When not in use a tight-fitting cap should always be placed over the mirror, and if a pad of cotton wadding of the same diameter is made to inlay this cap it tends to preserve the film by absorbing much of the moisture that otherwise condenses on its surface. The ‘Hints on Reflecting-Telescopes,’ by W. H. Thornthwaite and by G. Calver, and the ‘Plea for Reflectors,’ by J. Browning, may be instructively consulted by all those who use this form of instrument. The latter work is now, however, out of print, and Mr. Browning tells me that he has quite relinquished the manufacture of reflecting-telescopes. Mr. G. With of Hereford, who formerly supplied the mirrors for his instruments, has recently disposed of his reserve stock and entered an entirely different sphere of labour. In the publications above alluded to amateurs will find a large amount of practical information on the value and treatment of glass mirrors.

Opera-Glass.—A very useful adjunct, and often a really valuable one to the astronomical amateur, is the Opera-Glass, or rather the larger form of this instrument generally known as the Field-Glass. Of certain objects it gives views which cannot be surpassed, and it is especially useful in observations of variable stars and large comets. Whenever the horizon is being scanned for a glimpse of the fugitive Mercury, or when it is desired to have a very early peep at the narrow crescent of the young Moon, or to pick up Venus at midday, or Jupiter before sunset, all one has to do is to sweep over the region where the object is situated, when it is pretty sure to be caught, and the unaided eye will probably reach it soon afterwards. The opera-glass has the dignity of being the first telescope invented, for even its binocular form is not new; it is virtually the same pattern of instrument that was introduced at Middleburg in 1609, though its compound object-glasses are of more modern date. Anyone who entertains any doubts as to the efficacy of the opera-glass or has had little experience in its use will do well to look at the Pleiades and compare the splendid aspect of that cluster, as it is there presented, with the view obtained by the naked eye, and he will acknowledge at once that it constitutes a tool without which the observer’s equipment is by no means perfect. The object-glasses should have diameters of 2 or 2½ inches, and the magnifying power lie between 4 and 6. There is a large field of view and the images are very bright. The observer is enabled to enjoy the luxury of using both his eyes, and when he directs the instrument upon a terrestrial landscape he will be gratified that it does not turn the world upside down! It is not surprising that an appliance, with recommendations so significant, is coming more into favour every day, and for those branches suitable to its means it is doing much useful work. A volume has been recently published dealing expressly with the use of the opera-glass in Astronomy; and in the ‘Journal of the L.A.S.’ vol. vii. p. 120, there is an excellent paper by Major Markwick on the same subject. This instrument will never, of course, by the nature of its construction, be comparable to a modern telescope in regard to power, for Galilei, when he augmented his magnifiers to 30, appears to have practically exhausted the resources of this appliance. But in all those departments requiring an expansive field and little power with a brilliant and distinct image, the larger form of opera-glass is a great desideratum, and its portability is not one of the least of its advantages.

Dewing of Mirrors.—The disposition of mirrors to become clouded over upon rises of temperature is a point meriting comment. When permanently left in a telescope, fully exposed out of doors, the speculum undergoes daily transitions. The heat generated in the interior of the tube by the sun’s action causes a thick film of moisture to form upon the silvered surface of the mirror, which remains in this state for a considerable time, though the moisture evaporates before the evening. The flat is similarly affected, and the result of these frequent changes is that the coating of silver becomes impaired and presents a crackly appearance all over the surface. Sometimes when a marked increase of temperature occurs towards evening the speculum is rendered totally unserviceable until it has been submitted to what Dr. Kitchiner terms a process of “roasting.” The vapour will soon disappear when the mirror is brought indoors and placed before a fire; but it is not till some time after it has been remounted in the tube that it will perform satisfactorily. Those who keep their mirrors in more equable temperatures will not experience these inconveniences, which may also in some measure be obviated by regularly placing a tight-fitting cap, inlaid with cotton-wool, over the speculum at the conclusion of work. This also protects the silver from the yellow sulphurous deposit which soon collects upon it if used in a town. All sudden variations of temperature act prejudicially on the performance of specula, and their best work is only accomplished when free from such disturbing elements. I have rarely found the flat to become dewed in a natural way during the progress of observation. If on a cold night the observer puts his hand upon its supports in order to alter its adjustment it instantly becomes dewed, or if he stands looking down the tube it is almost sure to be similarly affected; but in the ordinary course of work the flat is little liable to become dewed in sensible degree. With refractors dew-caps are very necessary, though they do not always prevent the deposition of moisture on the object-glass, and this occasions frequent wiping or drying, which in either case is very objectionable.

Celestial Globe.—This forms another extremely useful addendum to the appliances of the amateur. It enables a great many problems to be solved in a very simple manner, and helps the young student to a lucid comprehension of the apparent motions and positions of the fixed stars. With ‘Keith on the Globes’ as a reference-book he may soon acquire the method of determining the times of rising, southing, and setting of any celestial object the place of which is known. He can also readily find the height (altitude) and bearing (azimuth) at any time. The distance in degrees between any two stars or between a star and the Moon, a planet, or a comet may be found at a glance by laying the quadrant of altitude on the pair of objects and reading off the number of degrees separating them. If a new comet has been discovered, its position should be marked in pencil upon the globe; and the observer, after having noted its exact place relatively to neighbouring stars, may proceed to identify the object with his telescope. If a large meteor is seen, its apparent path amongst the constellations should be projected on the globe and the points, in R.A. and Dec., of beginning and ending of the flight read off and entered in a book. In many other practical branches of astronomy this instrument will prove highly serviceable, and is far preferable to a star-atlas. But the latter is the most useful to the beginner who is just learning the names of the stars and the configuration of the chief groups, because on the globe the positions are all reversed east and west. The surface of the globe represents the entire star-sphere reduced to a common distance from the earth, and as seen from outside that sphere. The observer, therefore, must imagine his eye to be situated in the centre of the globe, if he would see the stars in the same relative places as he sees them in the heavens. The reversion of the star-positions to which we have been alluding is very confusing at first, and no doubt it provokes mistakes, but a little experience will practically remove this objection. The one great recommendation to a star-atlas is that it displays the stars in the natural positions in which they are discerned by the eye, thus enabling the student to become readily acquainted with them, whereas the celestial globe affords no such facility. But in other respects the latter possesses some valuable functions, and the amateur who devotes some of his leisure to mastering the really useful problems will attain a knowledge that will be of great benefit to him in after years. A globe of 12-inches diameter will be large enough for many purposes, but one of 18-inches will be the most effective size. It should be mounted on a tall stand with single body and tripod base. The stands, fitted with three parallel legs, in which the globe is supported in the middle by weak connections from them, are not nearly so durable. I have used several 18-inch globes mounted in this manner, and the supports have quite given way under the pressure of constant use; but this is impossible with the strong single body, which is capable of withstanding any strain. Globes are frequently to be obtained second-hand, and at trifling cost; but the observer must allow for precession if he uses an old article. Many of the stars will be 1° or 2° east of the positions in which they are marked on the globe; and it will be necessary to remember this if the appliance is to be employed for exact results.

Observatories.—Massive and lofty buildings have long gone out of fashion, and lighter, drier structures have properly supplanted them. Instruments of size are generally placed on or near the ground and solidly supported to ensure stability, while the other erections are made consistent with the necessity for pretty equable temperature and freedom from damp. Amateurs will ordinarily find that a simple wooden enclosure for the telescope, with suitable arrangements for opening the top in any direction, is sufficient for their purpose and very inexpensive. Some observers have, indeed, secured the desired shelter for themselves and their telescopes by means of a canvas tent provided with ready means for obtaining sky-room. Berthon has given a good description of an amateur’s observing-hut in ‘The English Mechanic’ for October 13th and 20th, 1871; and Chambers supplies some information about amateur observatories in ‘Nature’ for November 19th, 1885[8]. Mr. Thornthwaite’s. ‘Hints on Telescopes’ may be usefully consulted for details of the Romsey Observatory, which, like the Berthon model, seems peculiarly adapted to the necessities of the amateur. The great requirements in such structures are that they should be dry and not obstruct any region of the firmament. They should also be large enough to allow the observer perfect freedom in his movements and during the progress of his observations. They are then decided advantages, and will materially add to that comfort and convenience without which it is rarely possible to accomplish really good work. When an observatory is to be dispensed with it becomes necessary to erect a small wooden house near the instrument, especially if placed at the far end of a garden, in which the observer may keep certain appliances, such as a lantern, celestial globe, step-ladder or observing-seat, oil, &c. Here also he may record his seeings, complete his sketches, and consult his working-list, star-charts, and ephemerides. A shelter of this sort, apart from its practical helpfulness, avoids any necessity for the observer to go in and out of doors, up and down stairs, &c., to the annoyance of the rest of his family, who, on a frosty night, are decidedly not of an astronomic turn, and vastly prefer house-warming to stargazing!

[CHAPTER IV.]
NOTES ON TELESCOPIC WORK.

Preparation.—Working-Lists.—Wind.—Vision.—Records.—Drawing.—Friendly Indulgences.—Open-Air Observing.—Method.—Perseverance.—Definition in Towns.—Photography.—Publications.—Past and Future.—Attractions of Telescopic Work.

Preparation.—An observer in commencing work in any department of astronomy will find it a very great assistance to his progress if he carefully reads and digests all that has been previously effected in the same line. He will see many of the chief difficulties and their remedies explained. He will further learn the best methods and be in the position of a man who has already gained considerable experience. If he enter upon a research of which he has acquired no foreknowledge he will be merely groping in the dark, and must encounter many obstacles which, though they may not effectually turn him from his purpose, will at least involve a considerable expenditure of time and labour. On the other hand, a person who relies upon guidance from prior experimentalists will probably make rapid headway. He will be fortified to meet contingencies and to avoid complications as they arise. He will be better enabled to discriminate as to the most eligible means and will confidently endeavour to push them to the furthest extent. By adopting existing instructions for his direction and familiarizing himself with the latest information from the best authorities he will in a great measure ensure his own success or at least bring it within measurable] distance. The want of this foreknowledge has often been the main cause of failure, and it has sometimes led to misconceptions and imaginary discoveries; for after much thought and labour a man will overcome an impediment or achieve an end in a way for which he claims credit, only to find that he has been anticipated years before and that had he consulted past records, his difficulties would have been avoided and he might have pressed much nearer the goal. Too much importance cannot be attached to the acquisition of foreknowledge of the character referred to, though we do not mean that former methods or results are to be implicitly trusted. Let every observer judge for himself to a certain extent and let him follow original plans whenever he regards them as feasible; let him test preceding results whenever he doubts their accuracy. We recommend past experiences as a guide, not as an infallible precept. It would be as much a mistake to follow the old groove with a sort of credulous infatuation as it would be to enter upon it in utter ignorance of theoretical knowledge. An observer should take the direction of his labours from previous workers, but be prepared to diverge from acknowledged rules should he feel justified in doing so from his new experiences.

Fig. 18.

Refracting-Telescope on a German Equatoreal.

Working-Lists.—Full advantage should be taken of good observing weather. Sir John Herschel most aptly said that no time occupied in the preparation of working-lists is ill-spent. In our climate the value of this maxim cannot be overrated. If the 100 hours of exceptionally good seeing, available in the course of the year, are to be profitably employed, we must be continually prepared with a scheme of systematic work. The observer should compile lists of objects it is intended to examine, and their places must be marked upon the globe or chart so as to avoid all troublesome references during the actual progress of observation. If he has to consult ephemerides and otherwise withdraw attention from the telescope he loses valuable time: moreover the positions hurriedly assigned in such cases are frequently wrong and entail duplicate references, involving additional waste of time; all this may be avoided by careful preparation beforehand. If he has a series of double or variable stars to observe he must tabulate their places in convenient order so as to facilitate the work. If he intend hunting up nebulæ or telescopic comets he must carefully mark their positions relatively to adjoining stars. In the case of selenographical objects or planetary markings he may equally prepare himself by previous study. Adopting these precautions, objects may be readily identified and the work expedited. When no such preparation is made much confusion and loss of time result. On a cloudy, wet day observers often consider it unnecessary to make such provision and they are taken at a great disadvantage when the sky suddenly clears. A good observer, like a good general, ought to provide, by the proper disposition of his means, against any emergency. In stormy weather valuable observations are often permissible if the observer is prompt, for the definition is occasionally suitable under such circumstances. The most tantalizing weather of all is that experienced during an anti-cyclone in winter. For a week or two the barometer is very steady at a high reading, the air is calm, and the sky is obscured with an impenetrable mass of clouds.

Wind.—The influence of wind on definition has been much discussed in its various aspects, but it is scarcely feasible to lay down definite rules on the subject. The east wind is rarely favourable to good seeing, but the law is far from absolute. We must remember that several distinct currents sometimes prevail, and the air strata at various elevations are of different degrees of humidity and therefore exercise different effects upon telescopic definition. A mere surface breeze from the east may underlie an extensive and moist current from the south-west, and telescopic definition may prove very fair under the combination. Calm nights when there is a little haze and fog, making the stars look somewhat dim, frequently afford wonderfully good seeing. As a rule, when the stars are sparkling and brilliant, the definition is bad; planetary disks are unsteady and the details obliterated in glare. But this is not always so. I have sometimes found in windy weather after storms from the west quarter, when the air has become very transparent, that exceptionally sharp views may be obtained; but unfortunately they are not without drawbacks, for the telescope vibrates violently with every gust of wind and the images cannot be held long enough for anything satisfactory to be seen. The tenuous patches of white cirrous cloud which float at high altitudes will often improve definition in a surprising manner, especially on the Moon and planets. Of course this does not apply to nebulæ or comets, which are objects of totally different character and essentially require a dark night rather than good definition before they may be seen under the best conditions. As a rule, a steady, humid atmosphere is highly conducive to good seeing, and it is rather improved than impaired by a little fog or thin, white cloud. Some unique effects of peculiar definition, such as oval or triangular star disks, have been occasionally recorded, but we must content ourselves with a bare reference to these phenomena. With regard to the general question it may, however, be added that the character of the seeing often varies at very short intervals in this climate. In the course of a night’s work the definition will sometimes fluctuate in a most remarkable manner. An observer who comes to the telescope and finds it impossible to obtain satisfactory images should not entirely relinquish work at the first trial. After an interval he should again test its performance, for it frequently happens that a night ushered in by turbulent vapours, improves greatly at a later period, and in the morning part becomes so fine that it is worthy to be included in the select 100 hours assigned by Sir W. Herschel as the annual limit. Those who reside in towns will usually get the best definition after midnight, because there is less interference then from smoke and heated vapours. It would greatly conduce to our knowledge of atmospheric vagaries as affecting definition, if observers, especially those employing large aperture, preserved records as to the quality of the seeing, also direction of wind and readings of the barometer and thermometer.

Vision.—There are perhaps differences quite as considerable in powers of vision as in quality of definition. It is not meant by this that the same person is subject to great individual variations, though some people are certainly liable to fluctuations, according to state of health and other conditions. Some eyes, as already stated, are less effective in defining planetary markings than in detecting minute stars or faint satellites of distant planets. Of course the natural capacity is greatly enhanced by constant practice, for the human eye has proved itself competent to attain a surprising degree of excellence by habitual training. Frequent efforts, if not overpressed so as to unduly strain the optic nerves, are found to intensify rather than weaken the powers of sight. Thus a distinguishing trait among astronomers has been their keenness of vision, which, in many cases, they have retained to an advanced age. It is true Dr. Kitchiner said his “eye at the age of forty-seven became as much impaired by the extreme exertion it had been put to in the prosecution of telescope trials, as an eye which has been employed only in ordinary occupations usually is at sixty years of age!—to cultivate a little acquaintance with the particular and comparative powers of telescopes requires many extremely eye-teasing experiments.” But the Doctor’s opinion is not generally confirmed by other testimony, the fact being that the eye is usually strengthened by special service of this character. To unduly tax or press its powers must result in injury; but it is well known that the capacities of our sight and other senses are enhanced by their healthy exercise, and that comparative disuse is a great source of declining efficiency. Before the observer may hope to excel as a telescopist it is clear that a certain degree of training is requisite. Many men exhibit very keen sight under ordinary circumstances, but when they come to the telescope are hopelessly beaten by a man who has a practised eye. On several occasions the writer was much impressed with evidences of extraordinary sight in certain individuals, but upon being tested at the telescope they were found very deficient, both as regards planetary detail and faint satellites. Objects which were quite conspicuous to an experienced eye were totally invisible to them. I believe it is a good plan for habitual observers to employ method in exercising their sight. In my own case I invariably use the right eye on the markings of planets and the left on minute stars and satellites. Practice has given each eye a superiority over the other in the special work to which it has been devoted, and I fancy the practice might be more generally followed with success.

It is an advantage to keep both eyes open when in the act of observing, especially when surrounding objects are perfectly dark and there is no distracting light from neighbouring windows or lamps. The slight effort required to keep the disengaged eye closed interferes with the action of the other, and though this is but trivial, critical work is not efficiently performed under such conditions. Whenever light interferes the observer may exclude it by a shade so arranged as to afford complete protection to the unoccupied eye.

If faint objects are to be examined the observer should remain in a dark situation for some little time previously, so that the pupil of the eye may be dilated to the utmost extent and in a state most suitable for such work. After coming from a brilliantly lit apartment, or after viewing the Moon or a conspicuous planet, the eye is totally unfit to receive impressions from a difficult object, such as a minute star or faint nebula or comet; some time must be allowed to elapse so that the eye may recover its sensitiveness. As a rule amateurs will find it best to confine their attention to one class of objects only on the same evening, for if the Moon is first examined and then immediately afterwards the telescope is directed upon double stars and nebulæ, the latter objects are little likely to be seen with good effect. If faint objects generally are persistently studied night after night and the observer refrains from solar and lunar work, his eye will acquire greater sensitiveness and he will readily pick up minute forms which are utterly beyond the reach of a man who indiscriminately employs his eye and telescope upon bright and faint objects.

Records.—With regard to records, every observer should make a note of what he sees, and at the earliest possible instant after the observation has been effected. If the duty is relegated to a subsequent occasion it is either not done at all or done very imperfectly. The most salient features of whatever is observed should be jotted down in systematic form, so as to permit of ready reference afterwards. It is useful to preserve these records in a paged book, with an index, so that the matter can be regularly posted up. The negligence of certain observers in this respect has resulted in the total loss of valuable observations. Even if the details appear to possess no significance, they should be faithfully registered in a convenient, legible form, because many facts deemed of no moment at the time may become of considerable importance. The observer should never refrain from such descriptions because he attributes little value to them. Some men keep voluminous diaries in which there is scarcely anything worth record; but this is going to the other extreme. All that is wanted is a concise and brief statement of facts. Some persons have omitted references to features or objects observed because they could not understand them, and rather distrusted the evidence of their eyes; but these are the very experiences which require careful record and reinvestigation.

Drawing.—Few observers are good draughtsmen; but it is astonishing how seldom we meet with real endeavours to excel in this respect. Every amateur should practise drawing, however indifferent his efforts may be. Delineations, even if roughly executed, are often more effective than whole pages of description. Pictorial representations form the leading attraction of astronomical literature, and are capable of rendering it more interesting to the popular mind than any other influence. They induce a more apt conception of what celestial objects are really like than any amount of verbal matter can possibly do. For this reason it becomes the obvious duty of every observer to cultivate sketching and drawing, at least in a rudimentary way. He will frequently find it essential to illustrate his descriptions, so as to ensure their ready comprehension. In fact, a thoroughly efficient observer must of necessity become a draughtsman. It should, however, be his invariable aim to depict just what he sees and in precisely the form in which it impresses his eye. Mere pictorial embellishments must be disregarded, and he should be careful not to include doubtful features, possibly existing in the imagination alone, unless he intends them simply for his own guidance in future investigations. If he sees but little, and it is faithfully delineated, it will be of more real value than a most elaborate drawing in which the eye and imagination have each played a part. It is an undoubted fact that some of the most striking illustrations in astronomical handbooks are disfigured by features either wrongly depicted or having no existence whatever. There is very great need for caution in representing such markings only as are distinctly and unmistakably visible. In all cases where the object is new or doubtful the observer should await duplicate observations before announcing it. It is better that new features should evade discovery than that delusive representations should be handed down to posterity. As regards selenographical drawings I would refer the reader to what Mr. Eiger advises on p. 21 and 22 of volume v. of the ‘Journal of the Liverpool Astronomical Society.’ My own plan in sketching at the telescope is to first roughly delineate the features bit by bit as I successively glimpse them, assuring myself, as I proceed, as to general correctness in outline and position; then, on completion, I go indoors to a better light and make copies while the details are still freshly impressed on the mind. To soften details a small piece of blotting-paper must be wrapped round the pointed end of the pencil, and the parts requiring to be smoothed gently touched or rubbed until the desired effect is attained. This simple method, properly applied, will enable delicate markings to be faithfully reproduced, and it certainly adds in no small degree to the merit of a drawing.

Friendly Indulgences.—Every man whose astronomical predilections are known, and who has a telescope of any size, is pestered with applications from friends and others who wish to view some of the wonders of the heavens. Of course it is the duty of all of us to encourage a laudable interest in the science, especially when evinced by neighbours or acquaintances; but the utility of an observer constituting himself a showman, and sacrificing many valuable hours which might be spent in useful observations, may be seriously questioned. The weather is so bad in this country that we can ill spare an hour from our scanty store. Is it therefore desirable to satisfy the idle curiosity of people who have no deep-seated regard for astronomy, and will certainly never exhibit their professed interest in a substantial manner? Assuredly not. The time of our observers is altogether too valuable to be employed in this fashion. Yet it is an undisputed fact that some self-denying amateurs are unwearying in their efforts to accommodate their friends in the respect alluded to. My own impression is that, except in special cases, the observer will best consult the interests of astronomy, as well as his own convenience and pleasure, by declining the character of showman; for depend upon it a person who appreciates the science in the right fashion will find ways and means to procure a telescope and gratify his tastes to the fullest capacity. Some years ago I took considerable trouble on several evenings in showing a variety of objects to a clerical friend, who expressed an intention to buy a telescope and devote his leisure to the science. I spent many hours in explanations &c.; but some weeks later my pupil informed me his expenses were so heavy that he really could not afford to purchase instruments. Yet I found soon after that he afforded £30 in a useless embellishment of the front of his residence, and it so disgusted me that I resolved to waste no more precious time in a similar way.

Open-Air Observing.—Night air is generally thought to be pernicious to health; but the longevity of astronomers is certainly opposed to this idea. Those observers who are unusually susceptible to affections of the respiratory organs must of course exercise extreme care, and will hardly be wise in pursuing astronomical work out of doors on keen, wintry nights. But others, less liable to climatic influences, may conduct operations with impunity and safety during the most severe weather. Precautions should always be taken to maintain a convenient degree of warmth; and, for the rest, the observer’s enthusiasm must sustain him. A “wadded dressing-gown” has been mentioned as an effective protection from cold. I have found that a long, thick overcoat, substantially lined with flannel, and under this a stout cardigan jacket, will resist the inroads of cold for a long time. On very trying nights a rug may also be thrown over the shoulders and strapped round the body. During intense frosts, however, the cold will penetrate (as I have found while engaged in prolonged watches for shooting-stars) through almost any covering. As soon as the observer becomes uncomfortably chilly he should go indoors and thoroughly warm his things before a fire. He may then return fortified to his work and pursue it for another period before the frost again makes its presence disagreeably felt. On windy nights a knitted woollen helmet to cover the head, and reaching to the shoulders, is an excellent protection; but an observer had better not wear it more often than is imperative, or it becomes a necessity on ordinary nights. It is a great mistake to suppose that “a glass of something hot” before going into the night air is a good preventive to catching cold. It acts rather in the contrary way. The reaction after the system has been unduly heated only renders the observer more sensitive, and the inhalation of cold air is then very liable to induce affections of the throat.

A telescope permanently erected in the open, and exposed to all weathers, must soon lose its smart and bright appearance, but it need lose none of its efficiency, which is of far more importance; for it is intended for service, not for show. The instrument should be kept well painted and oiled. I find vaseline an excellent application for the screws and parts controlling the motions, as it is not congelative like common oils. The observer, before a night’s work and before darkness sets in, will do well to examine his instrument and see that it is in the best condition to facilitate work. Whole tribes of insects take up their habitation in the base or framework, and even in the telescope itself if they can effect a lodgment; and I have sometimes had to sweep away a perfect labyrinth of spiders’ webs from the interior of the main tube. On one occasion I could not see anything through the finder, try how I would. I afterwards discovered that a mason-wasp (Odynerus murarius) had adopted the vacuity in front of the eye-lens as a suitable site for her nest; and here she had formed her cells, deposited her eggs, and enclosed the caterpillars necessary for the support of the young when hatched. On another night I came hurriedly to the telescope to observe Jupiter with my single-lens eyepiece, power 252, but could make nothing out of it but a confused glare, subject to sudden extinctions and other extraordinary vagaries. I supposed that the branches of a tree, waving in the wind, must be interposed in the line of sight, but soon saw this could not possibly be the explanation. Looking again into the eyepiece, I caught a momentary glimpse of what I interpreted for the legs of an insect magnified into gigantic proportions and very distinct on the bright background formed by Jupiter much out of focus. On detaching the eyepiece and carrying it indoors to a light, an innocent-looking sample of the common earwig crawled out of it. The gyrations of the insect in its endeavours to find a place of egress from its confinement had clearly caused the effects alluded to. Telescopic observers are thus liable to become microscopic observers before they are conscious of the fact, and perhaps also in opposition to their intention. Other experiences might be narrated, especially as regards nocturnal observing in country or suburban districts, where the “serious student of the skies” may, like myself, find diversion to his protracted vigils by the occasional capture of a too-inquisitive hedgehog or some other marauding quadruped.

Fig. 19.

The Author’s Telescope: a 10-inch With-Browning Reflector.

Method.—Nearly all the most successful observers have been men of method. The work they took in hand has been followed persistently and with certain definite ends in view. They recognized that there should be a purpose in every observation. Some amateurs take an incredible amount of pains to look up an object for the simple satisfaction of seeing it. But seeing an object is not observing it. The mere view counts for nothing from a scientific standpoint, though it may doubtless afford some satisfaction to the person obtaining it. A practical astronomer, with his own credit at stake and the interests of the science at heart, will require something more. In observing a comet he will either fix its position by careful measurement with reference to stars near, or critically examine its physical peculiarities, or perhaps both. In securing these data he will have accomplished useful work, which may quite possibly have an enduring value. In other branches of observation his aim will be similar, namely to acquire new materials with regard to place or to physical phenomena, according to the nature of the research upon which he happens to be engaged. Such results as he gathers are neatly tabulated in a form convenient for after comparisons. There have been instances, we know, where sheer carelessness has resulted in the loss of important discoveries. Lalande must have found Neptune (and mathematical astronomy would have been robbed of its greatest triumph) half a century before it was identified in Galle’s telescope, but his want of care enabled it to elude him just when he was hovering on the very verge of its discovery. Numerous other instances might be mentioned. Failure may either arise from imperfect or inaccurate records, from a want of discrimination, from neglect in tracing an apparent discordance to its true source, or from hesitation. I may be pardoned for mentioning a case within my own experience. On July 11, 1881, just before daylight, I stood contemplating Auriga, and the idea occurred to me to sweep the region with my comet eyepiece, but I hesitated, thinking the prospect not sufficiently inviting. Three nights later Schæberle at Ann Arbor, U.S.A., discovered a bright telescopic comet in Auriga! Before sunrise on October 4 of the same year I had been observing Jupiter, and again hesitated as to the utility of comet-seeking, but, remembering the little episode in my past experience, I instantly set to work, and at almost the first sweep alighted upon a suspicious object which afterwards proved itself a comet of short period. These facts teach one to value his opportunities. They cannot be lightly neglected, coming as they do all too rarely. The observer should never hesitate. He must endeavour to at least effect a little whenever an occasion offers; for it is just that little which may yield a marked success—greater, perhaps, than months of arduous labour may achieve at another time.

Perseverance.—Persistency in observation, apart from the value derived from cumulative results, increases the powers of an observer to a considerable degree. This is especially the case when the same objects are subjected to repeated scrutiny. A first view, though it may seem perfectly satisfactory in its conditions and results, does not represent what the observer is capable of doing with renewed effort. Let us suppose that a lunar object with complicated detail is to be thoroughly surveyed. The observer delineates at the first view everything that appears to be visible. But a subsequent effort reveals other features which eluded him before, and many additional details are gradually reached during later observations. Ultimately the observer finds that his first drawing is scarcely more than a mere outline of the formation as he sees it at his latest efforts. Details which he regarded as difficult at first have become comparatively conspicuous, and a number of delicate structures have been exhibited which were quite beyond his reach at the outset. The eye has become familiarized with the object, and its powers fairly brought out by training and experience. This training is very serviceable, but is seldom appreciated in the degree of its influence. Many a tyro has abandoned a projected series of observations on finding that his initiatory view falls wofully short of published drawings or descriptions. He considers himself hopelessly distanced, and regards it as impossible to attain—much less excel—the results achieved by his predecessors. He does not realize that their work is the issue of years of close application, and that it represents the collective outcome of many successive nights. I need hardly say that it is a great mistake to anticipate failure in this way. No telescopic work has been done in the past that will not be done better in the future. No observer can rate his capacity until he has rigorously tested it by experience. The eye must become accustomed to an object before it is able to do itself justice. Those who have been sedulously engaged in a certain research will, as a rule, see far more than others who are but just entering upon it—not from a natural superiority of vision, but because of the aptitude and power acquired by practice. No matter how meagre an observer’s primary attempts may be, he should by no means relax his efforts, but rather feel that his want of success must be remedied by experience. It is a common fault with observers that they leave too much to their instruments, and rely upon them for the results which really depend entirely upon their personal endeavours. A skilled workman will do good work with indifferent tools; for after all it is the character of the man that is evident in his results, and not so much the resources which art places in his hands.

Much also depends upon the feelings by which the amateur is actuated when he commences work. A few enter into it with a degree of energy and determination that knows no wearying and will accept no defeat. Others display a half-hearted enthusiasm, and are constantly doubting either their personal ability or their instrumental means. Many others, again, when the circumstances appear a little against them regard failure as inevitable. It need hardly be said, however, that every difficulty may be surmounted by perseverance, and that a man’s enthusiasm is often the measure of his success, and success is rarely denied to him whose heart is in his work.

Definition in Towns.—The astronomical journals contain some interesting references to the definition of telescopes in large towns. Of course the purer the air the better for observational purposes. But observers who reside in populous districts need not despair of doing really useful work. The vapours hanging over a large city are by no means so objectional as is commonly supposed. When they are circulating rapidly across the observer’s field of view they will prove very troublesome at times; but in a comparatively tranquil state of the air definition is excellent. I have frequently found planetary markings very sharp and steady through the smoke and fog of Bristol. The interposing vapours have the effect of moderating the bright images and improving their quality. When there is a driving wind, and these heated vapours from the city are rolling rapidly past, objects at once appear in a state of ebullition, and the work of observation may as well be postponed. Smoke from neighbouring chimneys is utterly ruinous to definition: a bright star is transformed into a seething, cometary mass, and the planets undergo contortions of the most astonishing character. Large instruments being more susceptible to such influences—and, indeed, to atmospherical vagaries of all kinds—are chiefly affected by the drawbacks we have alluded to; but there are many opportunities when their powers may be fully utilized. In sweeping for faint comets, or in other work (such as the observation of nebulæ) where a dark sky is the first essential, a town station has a manifest disadvantage because of the artificial illumination of the atmosphere. But for general telescopic work the conditions do not offer a serious impediment, especially if the observer is careful to seize the many suitable occasions that must occur. The direction of the wind relatively to his position and the central part of the city, will occasion considerable differences to an observer who uses a telescope in a suburban locality.

Photography.—Upon this branch of practical astronomy not much will be said in this volume, as it is rather beyond its scope, and possibly also beyond the resources of ordinary amateurs, so far as really valuable work is concerned. A reference must, however, be made to an innovation which has deservedly assumed a very prominent place, and is clearly destined to exert an accelerating influence on the progress of exact astronomy. At present it is impossible to foretell how far it may be employed and extended, but judging from recent developments its applications will be as manifold as they will be valuable. Photographic records possess a great advantage over others, because they are more accurate and therefore more reliable. They are pictures from Nature taken by means free from the bias and error inseparable from mere eye-estimations or hand-drawings. The latter are full of discordances when compared one with another, and can seldom be implicitly trusted; but in the photograph a different state of things prevails. Here we have a faithful portrayal or reproduction of the object impressed by itself upon the plate. Hence it can be depended upon, because there has been no intermediate meddling either with its position or features by what may be termed artistic misrepresentation. True, there may be imperfections in the process; trifling flaws and obstructions will invariably creep in wherever comparatively new and novel work is attempted, but these will but little detract from the value of its results. Photography is obviously a means of discovery as well as a means of accurate record; for nebulæ and faint stars quite invisible to the eye have been distinguished for the first time upon the negatives. Those of our amateurs who intend working in this branch will find it a productive one, and not decaying in interest; but the necessary outfit will be expensive if thoroughly capable instruments are to be employed in the service.

STANMORE OBSERVATORY.
OUTSIDE VIEW

Publications.—The observer of to-day may esteem himself particularly fortunate in regard to the number and quality of the astronomical journals within his reach. Discoveries and current events receive prompt notice in these, and readers are fully informed upon the leading topics. Among the best of the periodicals alluded to are ‘The Observatory’ (Taylor & Francis, London), ‘The Sidereal Messenger’ (Northfield, Minn., U.S.A.), and L’Astronomie (Gautier-Villars, Paris). The Astronomische Nachrichten (Kiel, Germany) is a very old and valued serial, and ‘The Astronomical Journal’ (Cambridge, Mass., U.S.A.) may also be favourably mentioned. The ‘Monthly Notices’ of the Royal Astronomical Society and the ‘Journals’ of the Liverpool and British Astronomical Societies contain many interesting materials. ‘Nature,’ ‘The English Mechanic,’ and ‘Knowledge’ are among the English journals which devote part of their space to the science; and the beautiful illustrations in the latter entitle it to special recognition. It is evident, from this short summary, the amateur will find that his literary appetite may be amply satisfied, and should he desire a channel for recording his own work or ideas the publications referred to offer him every facility and encouragement.

As to almanacks, the ‘Nautical’ which has been termed “The Astronomer’s Bible,” includes a mass of tabular matter, some portion of which is of utility to the amateur, but it does not give data which are to be found in some other publications. I refer particularly to ephemerides of the satellites of Mars, Saturn, Uranus, and Neptune, to the dates of max. and min. of variable stars, to the times of rising and setting of the Sun, Moon, and planets, to the epochs and positions of meteor-showers, &c. The annual ‘Companion to the Observatory’ furnishes most of these details, and ‘Whitaker’s Almanack’ and Brown & Sons’ ‘Nautical Almanack’ each contain a large amount of serviceable information. The latter, however, is chiefly devoted to topics connected with Navigation, while ‘Whitaker’s Almanack’ is an extensive repertory of general facts.

With respect to handbooks much depends upon the direction of the observer’s labours, for he will obviously require works dealing expressly with his special subject. As a reliable companion to the telescope, Webb’s ‘Celestial Objects for Common Telescopes’ (4th edit., 1881) is indispensable; as a work of reference, and one forming an exhaustive conspectus of astronomical facts, Chambers’s ‘Descriptive Astronomy’ (4th edit., in 3 vols., 1889) may be recommended. Ledger’s ‘The Sun, its planets and their satellites’ is another good descriptive work. The beginner will find Noble’s ‘Hours with a 3-inch Telescope’ full of very instructive and agreeable material; while the more experienced astronomer, requiring a masterly exposition of the principles of the science, must procure Sir J. Herschel’s ‘Outlines’ (11th edit., 1871). In departmental work books of more exclusive character will be necessary. Thus, students of solar physics will want Young’s volume on ‘The Sun;’ observers of our satellite will need Neison’s ‘Moon.’ Those who find double stars interesting should get Crossley, Gledhill, & Wilson’s ‘Handbook’ and Chambers’s revised edition of Admiral Smyth’s ‘Cycle;’ others working on variable stars will need the Catalogues of Chandler and Gore. Jovian phenomena are well represented in Stanley Williams’s ‘Zenographic Fragments.’ Comets have been fully treated of in works by Cooper, Hind, and Guillemin; while to the observer of eclipses Johnson’s ‘Eclipses Past and Future’ is a valuable guide. Everyone interested in nebulæ will of course require Herschel-Dreyer’s ‘General Catalogue,’ containing 7840 objects and published by the Royal Astronomical Society in 1888. As to planetary observations, the several works of Webb, Chambers (vol. i.), and Ledger, first cited, supply a large amount of detail, almost obviating the necessity for further books.

Past and Future.—Observers and telescopes go on increasing day by day, and the future of astronomy has a most brilliant outlook. Photography has latterly effected a partial revolution in observation, though it can never entirely supersede old methods. Spectrum analysis, too, has formed a valuable acquisition during the last quarter of a century. With the new and refined processes, and with the gigantic instruments which have been erected, we may confidently anticipate many additions to our knowledge, especially in regard to very small and faint bodies which the inferior appliances of previous years have failed to grasp. And it is certain that some of the presumed discoveries of past times must be expunged, because not verified by the more perfect and powerful researches of a later date. Let us place in parallel columns (1) a few of the suspected objects thus to be erased, and (2) some of those which the future will probably add to our store:-–

Whatever may be the direction of future enquiries or the departures from old and tried methods, ordinary amateurs with small instruments, though handicapped more heavily as regards the prospect of effecting discoveries, may yet always be expected to accomplish useful work. Even to him who simply makes the science a hobby and a source of recreation in a leisure hour after the cares of business, the sky never ceases to afford a means of agreeable entertainment. He may neither achieve distinction nor seek it; but this he will assuredly do—gain an instructive insight into the marvellous works of his Creator, and acquire a knowledge which can only exercise an elevating tone to his life. The observer who quietly, from his cottage window, surveys the evening star or the new Moon through his little telescope often finds a deeper pleasure than the proficient astronomer who, from his elevated and richly appointed observatory, discovers new orbs with one of the most powerful instruments ever made.

Attractions of Telescopic Work.—In concluding our comments we may briefly refer to the importance and pleasure attached to telescopic work, and the growing popularity of observation in the attractive and diverse field of astronomy. A telescope may either be employed as an instrument of scientific discovery and critical work, or it may be made a source of recreation and instruction. By its means the powers of the eye are so far assisted and expanded that we are enabled to form a clearer conception of the wonderful works of the Creator than could be obtained in any other way. Objects which appear to natural vision in dim and uncertain characters are resolved, even in telescopes of the smallest pretentions, into pictures of well-defined outlines containing details of configuration far exceeding what are expected. And it is entirely owing to the exact measurements obtained under telescopic power that many of the most important problems of astronomy have been satisfactorily solved. To this instrument we are indebted, not only in a great measure for our knowledge of the physical features of many celestial bodies, but also for the accurate information we have gained as to their motions, distances, and magnitudes. Apart from this it is capable of affording ample entertainment to all those who are desirous of viewing for themselves some of the absorbing wonders of astronomy as described in our handbooks. And a demonstration of this practical kind is more effective than any amount of description in bringing home to the comprehension of the uninitiated the unique and picturesque side of astronomy.

[CHAPTER V.]
THE SUN.

Solar Observations.—Early notices of Spots.—Difficulties of the old observers.—Small instruments useful.—Tinted glass.—Solar Diagonal.—Structure of a Spot.—Methods of Drawing.—Ascertaining Dimensions.—Observer’s aims.—Eclipses of the Sun.—Periodicity of Spots.—Crateriform structure.—“Willow-Leaves.”—Rotation of the Sun.—Planetary bodies in transit.—Proper motion of Sun-spots.—Rise and decay of Spots.—Black Nuclei in the umbræ.—Bright objects near the Sun.—Cyclonic action.—Sudden outbursts of Faculæ.—Shadows cast by Faculæ.—Veiled Spots.—Recurrent disturbances.—Recurrent forms.—Exceptional position of Spots.—The Solar prominences.

“Along the skies the Sun obliquely rolls,

Forsakes, by turns, and visits both the poles;

Diff’rent his track, but constant his career,

Divides the times, and measures out the year.”

The Sun is not an object comprehended in the title of this volume. But to have omitted reference to a body of such vast importance, and one displaying so many interesting features to the telescopic observer, would have been inexcusable. We may regard the Sun as the dominant power, the controlling orb, and the great central luminary of our system. The phenomena visibly displayed on his surface assume a particular significance, as affecting a body occupying so high a place in the celestial mechanism.

The mean apparent diameter of the Sun is 32′ 3″·6, and his real diameter 866,000 miles. The apparent diameter varies from a minimum of 31′ 32″ at the end of June to a maximum of 32′36″; at the end of December; and the mean value is reached both at the end of March and September. The Sun’s mean distance from the Earth is about 92,900,000 miles, computed from a solar parallax of 8″·8, which appears to agree with the best of recent determinations. At this distance the linear value of 1″ of arc is 447 miles.

The Sun’s apparent diameter is as follows on the first day of each month:—

Solar observations may be pursued with a facility greater than that attending work in some other departments of practical astronomy. The Moon, planets, and stars have to be observed at night, when cold air, darkness, and other circumstances are the cause of inconvenience; but the student of the Sun labours only in the light and warmth of genial days, when all the incidentals to observation may be agreeably performed. There are, however, some drawbacks even in this pleasant sphere of work. The light of the Sun is so great that much persistent observation is apt to have an injurious effect on the eye, and will certainly deaden its sensitiveness on faint objects. In the summer months the observer experiences discomfort during a lengthy observation from remaining so long in the powerful rays of the Sun, some of which must fall upon his face unless measures are adopted to shield it. During the progress of solar work the student should always provide for himself as much shelter as possible from the glare, which must otherwise disturb that equanimity of feeling in the absence of which no delicate research is likely to be successfully conducted.

“Spots on the Sun” were remarked long before the telescope came into service. In the early Chinese annals many references are made to these objects; thus, in A.D. 188, February 14, it is recorded—“The colour of the Sun reddish-yellow; a fleckle in the Sun (bird-shaped).” Other ancient notices compare the spots to a flying bird, an apple, or an egg. Many spots were seen in later years, especially in 321, 807, 840, 1096, &c. In 807 a large black spot upon the Sun was watched during a period of eight days. It reflects much credit upon observers of a past age that they performed so many useful feats of observation, though relying simply upon the powers with which Nature alone had endowed them. They anticipated the telescope in some important discoveries. Large sun-spots are not, it is true, difficult features to perceive with the naked eye under certain circumstances; for whenever there is a fog or haze sufficiently dense to veil the lustre of the Sun in suitable degree, they can be readily seen, presuming, of course, that such spots are in existence at the time. They are sometimes observed, in a purely casual way, by people who may happen to glance at the Sun when he is involved in fog and looks like a dull, red ball suspended in the firmament. On one occasion, near sunset, in the autumn of 1870, I saw four large spots on different parts of the Sun, and these phenomena were very numerous at about this time. When spots attain a diameter of 50″ or more they may be detected by persons of good sight; but if the Sun is high and clear, coloured glass must be used to defend the eye.

Doubt hangs over the question as to the first telescopic observer of the spots. It is certain that Fabricius, Galilei, Harriot, and Scheiner all remarked them in about the year 1611; and of these Fabricius perhaps deserves the chief praise, as the first who published a memoir on the subject. Galilei appears undoubtedly to have had priority in recognizing the bright spots, or faculæ. Scheiner discovered that the black spots, or maculæ, are composed of a dark umbra and a fainter outlying shade, called the penumbra. Arago quotes him as having also described the Sun as “covered over its whole surface with very small, bright, and obscure points, or with lively and sombre streaks of very slender dimensions, crossing each other in all directions.” He announced, too, that the spots were confined to a narrow zone on the north and south sides of the equator, and this he termed the “Royal Zone.”

Some grave difficulties appear to have marked the attempts of the earlier observers; for they did not all use coloured glasses, and the dazzling light of the Sun, intensified by their lenses, often overpowered the sight, and so we find them awaiting opportunities when fog partly obscured the Sun near his rising or setting. Thus Harriot, who seems to have noticed and figured three sun-spots as early as 1610, Dec. 8, says:—“The altitude of the Sonne being 7 or 8 degrees, and it being a frost and a mist, I saw the Sonne in this manner.” His drawing followed. On another occasion he says:—“A notable mist: I observed the Sonne at sundry times, when it was fit.” Fabricius advised other observers to commence their observations by admitting only a small portion of the Sun into the field, so that the eye might be prepared to receive the light of the entire disk. Galilei was equally unaware of the advantage of tinted glass, and adopted the expedient of scanning the Sun when placed in the vicinity of the horizon. He remarks that “the spot of 1612, April 5 appeared at sunset;” and his writings contain other references of similar import. Scheiner, however, appears to have been more alive to the requirements of the work, and employed a plain green glass placed in front of the object-lens of his telescope.

Under the various circumstances we have been alluding to, the views obtained of the solar surface must necessarily have been of a very defective character, and the old observers at least deserve our sympathy in their exertions. No such obstacles confront the observer now. He has everything provided for him. Instrumental devices rob the Sun of his noonday brilliancy, and the eye serenely scans the details of his expansive image without the slightest pain or effort.

Small telescopes are peculiarly well adapted for solar observations. A good 3-inch refractor or 4-inch reflector will reveal an astonishing diversity of structure in the spots, and show something of the complicated minutiæ of the general surface. If the aperture of either instrument is 2 inches more than that stated, so much the better; but further than this it is rarely advisable to go. When the objective or mirror exceeds a diameter of 5 or 6 inches a stop often improves the images, and even smaller instruments will perform better when a little contracted. Definition is here the point to be desired; of light we have a superabundance. But if the observer meditates a critical analysis of the detail, either of a single spot, of a group of spots, or of a small area of the luminous surface, then a fair amount of aperture should be used, because greater aperture means greater separating power, and the latter will be useful in resolving the network of fibrous materials of which apparently the whole surface is composed. But for the common requirements of the observer an instrument of 3 or 4 inches will be found very effective, and it can either be used on a short tripod stand, placed on a steady table near a window having a south aspect, or it may be mounted on a tall garden stand and, according to the owner’s pleasure, either fixed at his window or in his garden. Two powers will be really necessary—one of about 60 and a field of quite 33″ to contain the entire disk and give a good general view, and another of 150 to which the observer will have recourse when examining details. Additional eyepieces will be sometimes useful, especially one of about 100; but the power of 60 previously recommended will, if a Huygenian, answer the same purpose, for if the field-lens is removed it will be increased to about 90. And should the observer think that anything is to be gained by a higher magnifier than 150, let him use the eye-lens only of that power. I have obtained many exquisite views of sun-spots with a single lens, and, instead of purchasing new eyepieces, a real advantage will be derived in adopting the plan suggested. There will be a smaller field and more colour about the image, but the improvement in definition is considerable, and more than balances these disadvantages.

Tinted glass must always be employed, unless a dense fog prevails, in which case the example of the old observers may be emulated. Several coloured glasses, of various depths, are needed for use according as the occasion requires. With a high Sun on a bright June day a darker tint will be necessary than in the winter, when the Sun’s rays are but feebly transmitted through the horizontal vapours. Red glass is unsatisfactory, as there is much heat and glare with it; but when used in combination with green the effect is excellent. Green alone is often used, and answers well; but it is not always thick and dense enough for the purpose. The plan of Sir W. Herschel, to interpose a glass trough of diluted ink, has never become popular, though he found it to succeed admirably. Smoked glass is also adapted for solar work, and recommends itself as being always obtainable at a minute’s notice. Some observers use a Barlow lens, with a thin film of silver deposited on the surfaces. It is then sufficiently transparent to give a neutral tint when held before a light, and sharp definition is said to be obtained without additional protection. Mr. Thornthwaite has also employed a coloured Barlow lens with effect.

A solar diagonal is a very necessary appliance if the observer would ensure perfect safety; for any refractor exceeding 2-inches aperture may, when turned on the Sun, focus enough heat to fracture the tinted sun-glass. The diagonal, by preserving a part only of the solar rays which are transmitted by the object-glass, enables observations to be made in security. This little instrument is comparatively cheap, and no telescope is complete without one. Dawes’s solar eyepiece serves the same purpose in a different manner, but it is an expensive luxury. In the latter construction there is a perforated diaphragm fixed near the eyepiece and so arranged that the quantity of admitted light may be modified consistently with the observer’s wishes.

In reflecting-telescopes with glass mirrors, effective views of the Sun are obtainable by employing unsilvered mirrors; for sufficient light is reflected by the glass surfaces to form good images of solar detail.

What, perhaps, interferes more than any other circumstance with successful observation of the Sun, is the fact that the rays, falling upon the telescope and objects near, induce a good deal of radiation, the direct tendency of which is to impair the definition and give a rippling effect to the disk. This is sometimes present in such force that the spots are subject to an incessant commotion, which serves to obliterate their more delicate features. A shady place is best, therefore, for such work; and if the observer leaves his telescope for a short time, intending to resume observations, it should never be placed broadside to the Sun, or the tube wall get hot, and heated currents must be generated in the interior, to the ruin of subsequent views.

A large sun-spot consists of an apparently black nucleus, a brown umbra, divided possibly by veins of bright matter or by encroachments of the penumbra which surrounds it. The latter is of much lighter tone than the umbra, though often similar in its general form. The outer edges of the umbra are serrated or scalloped by rice-grain protuberances. The inner region of the penumbra is much brighter than the outer, and the latter often exhibits quite a dusky fringe, induced by lines of dark material intervening with the brighter particles. The filaments forming the penumbra—often grouped in a radial manner with reference to the centre of a spot—would appear to be more widely separated near the outer border of the penumbra, and sufficiently so to allow sections of the umbral layer of the Sun to be observed through the interstices. The lighter tint of the interior part of the penumbra is stated to be due to contrast; but this is a mistake. The difference is too definite and distinct to permit such an explanation. Mr. Maunder says “that usually (not invariably) the penumbra darkens towards the umbra, and that the phenomenon as ordinarily described is merely an effect of contrast.” My own observations, however, appear to show that there is an actual difference of detail in the outer and inner portions of the penumbra, which gives a darker tone to the former.

In drawing the forms of sun-spots the observer must be expeditious, because of the variations which are quickly and constantly affecting them. In concluding a sketch I find it essential to make several alterations in it, owing to the changes which have occurred in the spots during the interval of a quarter of an hour or so since it was commenced. The details must be filled in consecutively, each one being the result of a careful scrutiny. When finished, the whole sketch should be compared with the object itself and amended if found necessary. The observer should also mark upon the sheet the measured or estimated latitude and longitude of the spot, and make a finished drawing from the basis of his sketch as soon as possible afterwards. At Stonyhurst Observatory excellent delineations of solar phenomena are made; and the late Father Perry, who lost his life in the cause of science, thus described the method:—“On every fine day the image of the Sun is projected on a thin board attached to the telescope, and a drawing of the Sun is made, 10½ inches in diameter, showing the position and outline of the spots visible. It is the first duty of the assistant who makes the drawings to note the position of the spots, and sketch their outlines. He then proceeds to shade in the penumbra and to draw the finer details, comparing the drawing from time to time by placing it alongside the projected image of the spots. The position of the faculæ is then filled in with a red pencil, so that the eye can at once recognize their grouping with respect to sun-spots, and the other details drawn with a black pencil.” The same astronomer also stated that, “as a general rule, careful drawings of the projected image of the Sun give much more satisfactory pictures of the solar surface than the photographs taken even at our best observatories. It is quite true that occasionally an exquisite photograph on an enlarged scale may be obtained, which exhibits features such as no pencil could portray as accurately, but rarely indeed will the photograph furnish all the details that a practised eye and hand, kept patiently at the sketch-board, will detect and faithfully describe. And the reason is not far to seek; for any experienced observer knows that, even on the finest day, the definition is continually changing with the sky, and that it is only at comparatively rare moments we can expect those perfect conditions that enable the finest details to stand out sharply, as Schiaparelli expresses it, like the faintest lines of a steel engraving. A photograph may be accidentally taken during one of these exceptionally favoured moments; but a patient draughtsman is almost sure to secure several of these best opportunities at each prolonged visit to his sketch-board. What would, therefore, be a great acquisition at present is a series of careful solar drawings, taken at short intervals of time, on days when characteristic spots are visible upon the Sun; and this would be the surest way of adding much valuable information to that already possessed concerning the changes that take place in the solar photosphere.”

With regard to ascertaining the dimensions of sun-spots, very precise results require accurate means of measurement and some mathematical knowledge. For the general purposes of the amateur, who will only want round numbers, simple methods may be adopted with success. I have used, on a 4-inch refractor, a graduated piece of plane glass, mounted suitably for insertion in the focus of the eyepiece, and marked with divisions 1/200 of an inch apart. With power 65 I find the Sun’s disk at max. distance covers 83 divisions of the graduated lens; so that one division = 22″·8, the Sun’s min. diameter being 1892″. Each division, therefore, is equal to 10,434 miles, the Sun’s real diameter being 866,000 miles.

Fig. 20.

Sun-spot of June 19, 1889, 2^h P.M.

I viewed a large spot on June 19, 1889, and found its major axis covered 2·6 divisions, = 59″·3[9]; so that its apparent length was about 27,000 miles. For

1892″:866,000 miles :: 59″·3:27,143 miles.

The same method may be adopted if the image is thrown upon a screen.

Approximate values are to be obtained by means of fine cross wires fixed in the eyepiece. Note the exact interval occupied by the Sun in crossing the vertical wire, and also the interval occupied by the large spot or group. If the Sun is 133 seconds in passing the wire, and the group 6·5 seconds, then

133 seconds:866,000 miles :: 6·5 seconds:42,323 miles.

This plan is likely to be most successful when the Sun is near its meridian passage; but it may be applied at any hour, if care is taken to adjust the eyepiece so that the Sun’s motion is precisely at right angles to the vertical wire. One other plan may be mentioned. Draw on cardboard, with compasses, a circle about 10 or 12 inches diameter, and divide this with 31 parallel lines. Subdivide each of the spaces into 5, less prominently marked. Then, during observation, keep both eyes open, and hold or fix the circular disk at a distance enabling it to coincide with the telescopic image of the Sun. By carefully noting how many divisions the group covers on the cardboard, its dimensions may be readily found, because one division will be equal to about 5410 miles. Of course these methods[10] are simply approximate, and only strictly applicable to objects not far removed from the central regions of the Sun, because the spots are portions of a sphere, and not angles subtended by a flat surface. When close to the E. or W. limbs, foreshortening is considerable, though the polar diameter of a spot is not affected by it then.

Presuming an observer to have his 3-or 4-inch telescope duly fitted with a solar diagonal and tinted glass, he may naturally ask, after his curiosity has been satisfied by the contemplation of his first sun-spot, what he can do further: What special features is he to look for? What changes ought to be recorded? What are the doubtful points that require to be cleared up as regards the Sun’s physical appearance? In what way are new and novel facts likely to be glimpsed? In a word, he desires to know in what manner he may employ his eyes and instrument usefully for science, while also gaining pleasure for himself. Information like this is often needed by the young student, and sometimes indeed by men who have already gained a little experience, and who possess much larger instruments than we have intimated above. In endeavouring to offer suggestions in response to such inquiries, I would remark that the nature and direction of a research essentially depend upon several conditions, viz. the observer’s inclination, his instrumental equipment, his place of observation, and the amount of time he can devote to the pursuit of his object. There are very few men who, like Schwabe of Dessau, will confront the Sun on nearly every day for more than forty years in order to learn something of its secrets. Such extraordinary pertinacity is fortunately not required, except in special cases. Amateurs may effect much valuable work in the short intervals which many of them steal either from business or domestic ties and offer at the shrine of astronomy.

There are quite a considerable number of attractive phenomena and features on which the solar observer will find ample employment, and to the principal of these it may be as well to make individual references.

Eclipses of the Sun.—These phenomena deservedly rank amongst the most important and impressive events displayed by the heavenly bodies, and they are specially interesting to the possessors of small telescopes. Solar eclipses have been so often made the subject of observation and discussion, that our knowledge of the appearances presented may be considered to be nearly complete. The various aspects of Nature on such occasions have been so attentively studied in their manifold bearings, that virtually nothing remains for the ordinary observer but to reexamine and corroborate facts already well ascertained. He can expect to glean few materials in a field where a plentiful harvest has just been reaped. But the eclipsed Sun, if it has revealed most of its secrets to previous investigators, has certainly not declined in attractiveness; and the amateur will find the spectacle still capable of exhibiting features which, though not full of the charms of novelty, will be sufficiently striking and diversified to be remembered long after the event has passed.

Fig. 21.

Solar Eclipses visible in England, 1891 to 1922.

Fig. 22.

Total Solar Eclipse of August 19, 1887.

Eclipses recur in cycles of 18 years and 10 days (= 6585 days). This period was determined by the ancients, and called the saros. By its means the times and magnitudes of eclipses were roughly computed long before astronomy became an exact science.

A solar eclipse is really an occultation of the Sun by the Moon; for the word eclipse, in its usual reference, denotes the obscuration of one body by its immersion in the shadow of another. During any single year there are never less than two eclipses, nor more than seven. Whenever there are two only, both are solar.

Since the fine solar eclipse of December 22, 1870, no large eclipse of the Sun has been visible in England. It is remarkable that during the thirty years from 1870 to 1900 these phenomena are all of an unimportant, minor character. Within the thirty years following 1891 there will be twelve solar eclipses, for which the Rev. S. J. Johnson has given projections (as shown on p. 98) for the period of greatest obscuration.

Total eclipses are extremely rare as regards their visibility at a given station. Thus between 878 and 1715 not one was observed at London, and during the next 500 years there will be a similar absence of such a phenomenon. The observer of total eclipses must perforce journey to those particular tracts of the earth’s surface over which the band of totality passes. On such occasions photography plays an important part; and the corona, the red flames, the shadow-bands, and numerous other features become the subjects of necessarily hurried observation and record, for totality endures for very few minutes[11].

As regards ordinary partial eclipses, amateurs usually find ample entertainment in noting the serrated aspect of the Moon’s contour projected on the bright Sun. It is also interesting to watch the disappearance and reappearance of the solar spots visible at the time. Rather a low magnifying power, with sufficiently expansive field to include the entire disk, is commonly best for the purpose of these observations.

Periodicity of Spots.—This detail may be said to have been fully investigated. Schwabe and Wolf have accomplished much in this direction. A work of this kind must, by the nature of it, extend over many years and entail many thousands of observations. It is therefore more suited to the professional astronomer than to the amateur, whose attention is more or less irregular owing to other calls. The sun-spot cycle is one of about 11 years, during which there are alternately few and many spots on the Sun. There appear to be some curious fluctuations, disturbing the regular increase and decrease in the number of spots; and these variations are worthy of more attention. The following are the years of observed maxima and minima of sun-spot frequency:—

These phenomena have been rare during the past few years. The next maximum may be expected in about 1894, when solar observers will probably have an abundance of new materials to study.

Crateriform Structure.—In 1769 Prof. Wilson, of Glasgow, while watching a sun-spot with a Gregorian reflecting-telescope, remarked that, as it approached near the limb, the penumbra became much foreshortened on the interior side. He inferred from this that the spots were cavities, and the idea has been generally accepted; so that these objects are sometimes termed solar craters, and commonly regarded as openings in the luminous atmosphere of the Sun. But the conclusion appears to be based on data not uniformly supporting it. In 1886 the Rev. F. Howlett published some observations which “entirely militate against the commonly received opinion that the spots are to any extent sunk in the solar surface as to produce always those effects of perspective foreshortening of the inner side of the penumbra (when near the limb) which have been described in various works on astronomy.” In a number of instances the penumbra is wider on the side nearest the Sun’s centre, whereas the converse ought to be the case on the cavity theory. The fine sun-spot of July 1889 offered an example of this; for when it was near the W. limb the W. side of the penumbra was obviously much narrower than the E. side, so that the appearance would indicate the object as an elevation rather than a depression. The observer should keep a register of the aspect of all pretty large spots near the limb, and note the relative widths of the E. and W. sides of the penumbra. An extensive table of such results would be interesting, and certain to throw some light on the theory of spot-structure. It is of course possible that occasionally the inner side of the penumbra is broader than the outer, and thus appears wider even on the limb, though really forming the side of a shallow depression.

“Willow-Leaves.”—In 1861 the late Mr. Nasmyth announced that the entire solar surface was composed of minute luminous filaments in the shape of “willow-leaves,” which interlaced one another in every possible variety of direction. This alleged discovery only met with doubtful corroboration. The objects were stated by some authorities to be simply identical with the “corrugations” and “bright nodules” of Sir W. Herschel. Mr. Stone called them “rice-grains.” The eagle-eyed Dawes thought “granulations” a more appropriate term, as it implied no consistency of form and size. Secchi referred to them as oblong filaments, and “rather like bits of cotton-wool of elongated form.” The Rev. F. Howlett described the Sun as presenting a granulated, mottled appearance in a 3-inch Dollond refractor, and mentioned that on the morning of June 9, 1865, the aspect of its surface was like that of new-fallen snow, the objects “being not rounded but sharply angular.” The opinions of observers were thus singularly diverse, and the result of several animated discussions at the Royal Astronomical Society was that little unanimity was arrived at, except as to the fact that the Sun’s surface was crowded with small luminous filaments of elongated form, and either rounded or angular at the ends. There was no accord as to their precise forms or distinctive manner of grouping. Some of the observers averred that the “willow-leaves” or “rice-grains” had no title whatever to be regarded as a new discovery, the same appearances having been recognized long before. Gradually the contention ceased, and though more than a quarter of a century has passed since the discussion arose there has been little new light thrown on the subject.

Amateurs will therefore do well to probe deeper into this promising branch of solar observation. As Mr. Nasmyth himself stated, considerable telescopic power is required, combined with a good atmosphere. But comparatively small instruments will also be useful, because of their excellent definition and efficacy in displaying details on a brilliant orb like the Sun. A power of 150 should be employed in examining small regions of the general surface, and also the edges of the umbra and penumbra of sun-spots. When definition is unusually sharp, and the details very distinct, the magnifying power should be increased if it can be done with advantage; and the observer should utilize an occasion like this to the utmost extent. On a really excellent day more may be sometimes detected than during several weeks when the atmosphere is only moderately favourable. The observations, being of a critical nature, should not be attempted in winter, when the Sun is low. I have frequently secured fine views of the delicate structure of the solar surface between about 8 and 9 A.M. in the summer months; and this is often a convenient time for amateurs to snatch a glimpse, before going to business.

With reference to the general question as to the existence of the “willow-leaves,” my conception of the matter is that the features described by Mr. Nasmyth are not new. His drawing of a spot in Sir J. Herschel’s ‘Outlines’ and Chambers’s ‘Descriptive Astronomy’ exhibits objects extremely uniform in shape and size, and this uniformity I have never observed in the penumbra of spots. As to the engraving in the ‘Outlines,’ showing the aspect of the interlaced “willow-leaves” on the general surface, this is also not realized in observation. The “corrugations” and “bright nodules” of Sir W. Herschel aptly represent what is seen, and they are possibly identical with the “very small bright and obscure points” and “lively and sombre streaks” of Scheiner, though seen much better and in more profusion of detail through the improved modern telescopes. The so-called “willow-leaves” are rounded at the ends, and are consistent neither in size nor shape. They encroach upon the umbra of the spots, and give a thatched appearance to the edges. The penumbra also shows this in its outer limits, where it is also fringed with lenticular particles. Drawings by Capocci and Pastorff seventy-five years ago, and published in Arago’s ‘Popular Astronomy,’ show the thatching at the edges of the umbra quite as palpably as it is represented in recent drawings.

Fig. 23.

Belts of Sun-spots, visible October 29, 1868.

Rotation of the Sun.—By noting when the same individual spots return to the same relative places on the disk, the approximate time of rotation is easily deduced. This varies according to the latitude of the spots[12]; whence it is evident the solar atmosphere is affected by currents of different velocities, causing the spots to vary in their longitudes with reference to each other. The Earth’s motion round the Sun causes the spots to travel apparently more slowly than they really do; for observations prove that a spot completes a rotation in 27 days 5 hours, whereas the actual time, after making allowance for the earth’s orbital motion, is about 25 days 7-3/4 hours. The period of rotation may be roughly found as follows, supposing a spot to return to precisely the same part of the disk in 27 days 5 hours:—

365^d 5^h 49^m + 27^d 5^h = 392 10^h 49^m.

Then

392^d 10^h 49^m (= 565,129^m) : 365^d 5^h 49^m (= 525,949^m)

:: 27^d 5^h (= 39,180^m) : 25^d 7^h 44^m (= 36,464^m).

For exact results several circumstances have to be considered, such as the direction of the spot-motions across the disk, as the chords vary according to the season; thus in June and December the spots traverse straight lines, while in March and September their paths are curved, like a belt on Saturn when the planet is inclined. Some of the spots display considerable proper motion; so that it is best to observe a number of these objects, and reduce the times to a mean result. They are not very durable, rarely lasting longer than a few weeks; but some of the more extensive disturbances are sustained for several months, during which many singular changes are effected. The period of rotation, as determined by several observers, is as follows:—

The motion of rotation is similar in direction to that in which the planets move found the Sun, namely from west to east. Hence the spots come into view on the east limb of the Sun, and disappear at the west.

Planetary Bodies in transit.—During observation the observer should particularly watch any very dark, small spots that may be visible, such as are isolated and pretty circular and definite in outline. If an object of this character is seen it should be examined with a high power, and its aspect critically noted. Should the observer entertain any suspicion of its being of a planetary nature, he should carefully determine its position on the disk, and, after a short interval, re-observe it for traces of motion. If it remains stationary, its true solar origin will be proved. If motion is shown, then the successive positions of the object during its transit, and its place of egress, with the time of each observation, should be recorded. In such a case it would be a good plan to project the Sun’s image, and mark the place of the suspicious object and chief sun-spots at short intervals. This would be more accurate than mere eye-estimation. The observer who scans the solar surface for intra-Mercurial planets must remember that, if any such bodies exist, they will probably be very diminutive. Venus, when on the Sun in December 1882, was a spot 63″ in diameter, and easily perceptible to the naked eye. Mercury, at the transits of 1861, 1868, and 1881, was a little less than 10″, but in 1878 was 12″. If “Vulcan,” the suspected interior planet, has any existence it may possibly be much smaller than Mercury, and will thus escape observation, unless the observer exercises great care in the search. The mobile, planetary spots asserted to have been seen on the Sun in past years prove nothing definite, and appear to have been illusory.

Proper Motion of Sun-spots.—This feature is one deserving more investigation. The distances separating individual spots should either be measured with a micrometer or determined by transits across a wire, and the displacement recorded from hour to hour or from day to day. Spots in different latitudes will almost certainly exhibit some change of relative place; and objects in the same latitude must be watched, for similar variations probably affect them. The physical peculiarities of such spots should be remarked, and also the alterations of appearance they undergo during the time they approach or recede from each other.

Rise and Decay of Spots.—Occasionally large spots are formed in an incredibly short time, and the disappearance of others has been equally sudden. Schwabe found, from many observations, that the western spots of a group are obliterated first; but authorities differ. I have usually observed that the smaller, outlying members of a group vanish before the larger spot, which then contracts and is invaded by tongues of faculæ; so that its effacement soon follows, and nothing remains to indicate the disturbance but bright ridges of faculæ, which are very conspicuous near the limb.

Black Nuclei in the Umbræ.—Dawes was the first to announce that the umbra sometimes included a much darker area or nucleus. This is present in nearly all large spots. A part of the umbra seems covered or veiled by a slightly luminous medium, and the portion unaffected looks black by contrast. On October 1, 1881, with a 2½-inch refractor, I saw a large sun-spot, the umbra of which was broken up into 7 fragments, and the S. preceding part appeared very black while the others showed a much lighter tint. In the fine spot of June 1889 a nucleus was also distinctly apparent; and this feature is sometimes so obvious in large spots that it may be observed with an instrument of only 2-inches aperture. I have usually remarked the nucleus on one side of the umbra, and abutting the penumbra. It may be formed by light patches of transparent material floating over the umbra, and leaving a part free where the Sun’s dark body is fully exposed. This light material is possibly suspended far above the umbra and inconstant in its position; so that the place and form of the nucleus should always be noted for traces of change. It is necessary that such details should be closely watched during an entire day, or several days; for the variations could then be followed, and perhaps reduced to some law. This persistence is very necessary, in order to solve many of the peculiarities of sun-spots, which, though pretty well known in appearance, have not been thoroughly studied in their various developments.