THE TELESCOPE

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Galileo’s Telescopes. (Frontispiece)
(Bull. de la Soc. Astron. de France.)

THE TELESCOPE

BY
LOUIS BELL, Ph.D.
CONSULTING ENGINEER; FELLOW, AMERICAN ACADEMY OF ARTS & SCIENCES; PAST-PRESIDENT,
THE ILLUMINATING ENGINEERING SOCIETY; MEMBER,
AMERICAN ASTRONOMICAL SOCIETY

First Edition

McGRAW-HILL BOOK COMPANY, Inc.
NEW YORK: 370 SEVENTH AVENUE
LONDON: 6 & 8 BOUVERIE ST., E. C. 4
1922

Copyright, 1922, by the
McGraw-Hill Book Company, Inc.

THE MAPLE PRESS YORK PA

[PREFACE]

This book is written for the many observers, who use telescopes for study or pleasure and desire more information about their construction and properties. Not being a “handbook” in two or more thick quartos, it attempts neither exhaustive technicalities nor popular descriptions of great observatories and their work. It deals primarily with principles and their application to such instruments as are likely to come into the possession, or within reach, of students and others for whom the Heavens have a compelling call.

Much has been written of telescopes, first and last, but it is for the most part scattered through papers in three or four languages, and quite inaccessible to the ordinary reader. For his benefit the references are, so far as is practicable, to English sources, and dimensions are given, regretfully, in English units. Certain branches of the subject are not here discussed for lack of space or because there is recent literature at hand to which reference can be made. Such topics are telescopes notable chiefly for their dimensions, and photographic apparatus on which special treatises are available.

Celestial photography is a branch of astronomy which stands on its own feet, and although many telescopes are successfully used for photography through the help of color screens, the photographic telescope proper and its use belongs to a field somewhat apart, requiring a technique quite its own.

It is many years, however, since any book has dealt with the telescope itself, apart from the often repeated accounts of the marvels it discloses. The present volume contains neither pictures of nebulæ nor speculations as to the habitibility of the planets; it merely attempts to bring the facts regarding the astronomer’s chief instrument of research somewhere within grasp and up to the present time.

The author cordially acknowledges his obligations to the important astronomical journals, particularly the Astro-physical Journal, and Popular Astronomy in this country; The Observatory, and the publications of the Royal Astronomical Society in England; the Bulletin de la Société Astronomique de France; and the Astronomische Nachrichten; which, with a few other journals and the official reports of observatories form the body of astronomical knowledge. He also acknowledges the kindness of the various publishers who have extended the courtesy of illustrations, especially Macmillan & Co. and the Clarendon Press, and above all renders thanks to the many friends who have cordially lent a helping hand—the Director and staff of the Harvard Observatory, Dr. George E. Hale, C. A. R. Lundin, manager of the Alvan Clark Corporation, J. B. McDowell, successor of the Brashear Company, J. E. Bennett, the American representative of Carl Zeiss, Jena, and not a few others.

Louis Bell.

Boston, Mass.,
February, 1922.

[CONTENTS]

THE TELESCOPE

[CHAPTER I]
THE EVOLUTION OF THE TELESCOPE

In the credulous twaddle of an essay on the Lost Arts one may generally find the telescope ascribed to far antiquity. In place of evidence there is vague allusion of classical times or wild flights of fancy like one which argued from the Scriptural statement that Satan took up Christ into a high mountain and showed him all the kingdoms of the earth, that the Devil had a telescope—bad optics and worse theology.

In point of fact there is not any indication that either in classical times, or in the black thousand years of hopeless ignorance that followed the fall of Roman civilization, was there any knowledge of optical instruments worth mentioning.

The peoples that tended their flocks by night in the East alone kept alive the knowledge of astronomy, and very gradually, with the revival of learning, came the spirit of experiment that led to the invention of aids to man’s natural powers.

The lineage of the telescope runs unmistakably back to spectacles, and these have an honorable history extending over more than six centuries to the early and fruitful days of the Renaissance.

That their origin was in Italy near the end of the thirteenth century admits of little doubt. A Florentine manuscript letter of 1289 refers to “Those glasses they call spectacles, lately invented, to the great advantage of poor old men when their sight grows weak,” and in 1305 Giordano da Rivalto refers to them as dating back about twenty years.

Finally, in the church of Santa Maria Maggiore in Florence lay buried Salvino d’Amarto degli Armati, (obiit 1317) under an epitaph, now disappeared, ascribing to him the invention of spectacles. W. B. Carpenter, F. R. S., states that the inventor tried to keep the valuable secret to himself, but it was discovered and published before his death. At all events the discovery moved swiftly. By the early fourteenth century it had spread to the Low Countries where it was destined to lead to great results, and presently was common knowledge over all civilized Europe.

It was three hundred years, however, between spectacles and the combination of spectacle lenses into a telescope, a lapse of time which to some investigators has seemed altogether mysterious. The ophthalmological facts lead to a simple explanation. The first spectacles were for the relief of presbyopia, the common and lamentable affection of advancing years, and for this purpose convex lenses of very moderate power sufficed, nor was material variation in power necessary. Glasses having a uniform focus of a foot and a half or thereabouts would serve every practical purpose, but would be no material for telescopes.

Myopia was little known, its acquired form being rare in a period of general illiteracy, and glasses for its correction, especially as regards its higher degrees, probably came slowly and were in very small demand, so that the chance of an optical craftsman having in hand the ordinary convex lenses and those of strong negative curvature was altogether remote. Indeed it was only in 1575 that Maurolycus published a clear description of myopia and hypermetropia with the appropriate treatment by the use of concave and convex lenses. Until both of these, in quite various powers, were available, there was small chance of hitting upon an instrument that required their use in a highly special combination.

At all events there is no definite trace of the discovery of telescopic vision until 1608 and the inventor of record is unquestionably one Jan Lippershey, a spectacle maker of Middelburg in Zeeland, a native of Wesel. On Oct. 2, 1608 the States-General took under consideration a petition which had been presented by Lippershey for a 30-year patent to the exclusive right of manufacture of an instrument for seeing at a distance, or for a suitable pension, under the condition that he should make the instrument only for his country’s service.

The States General pricked up its ears and promptly appointed on Oct. 4 a committee to test the new instrument from a tower of Prince Maurice’s palace, allotting 900 florins for the purchase of the invention should it prove good. On the 6th the committee reported favorably and the Assembly agreed to give Lippershey 900 florins for his instrument, but desired that it be arranged for use with both eyes.

Lippershey therefore pushed forward to the binocular form and two months later, Dec. 9, he announced his success. On the 15th the new instrument was examined and pronounced good, and the Assembly ordered two more binoculars, of rock crystal, at the same price. They denied a patent on the ground that the invention was known to others, but paid Lippershey liberally as a sort of retainer to secure his exclusive services to the State. In fact even the French Ambassador, wishing to obtain an instrument from him for his King, had to secure the necessary authorization from the States-General.

Bull. de la Soc. Astron. de France.
Fig. 1.—Jan Lippershey, Inventor of the Telescope.

It is here pertinent to enquire what manner of optic tube Lippershey showed to back up his petition, and how it had come to public knowledge. As nearly as we may know these first telescopes were about a foot and a half long, as noted by Huygens, and probably an inch and a half or less in aperture, being constructed of an ordinary convex lens such as was used in spectacles for the aged, and of a concave glass suitable for a bad case of short sightedness, the only kind in that day likely to receive attention.

It probably magnified no more than three or four diameters and was most likely in a substantial tube of firmly rolled, glued, and varnished paper, originally without provision for focussing, since with an eye lens of rather low power the need of adjustment would not be acute.

As to the invention being generally known, the only definite attempt to dispute priority was made by James Metius of Alkmaar, who, learning of Lippershey’s petition, on Oct. 17, 1608, filed a similar one, alleging that through study and labor extending over a couple of years he, having accidentally hit upon the idea, had so far carried it out that his instrument made distant objects as distinct as the one lately offered to the States by a citizen and spectacle maker of Middelburg.

He apparently did not submit an instrument, was politely told to perfect his invention before his petition was further considered, and thereafter disappears from the scene, whatever his merits. If he had actually noted telescopic vision he had neither appreciated its enormous importance nor laid the facts before others who might have done so.

The only other contemporary for whom claims have been made is Zacharius Jansen, also a spectacle maker of Middelburg, to whom Pierre Borel, on entirely second hand information, ascribed the discovery of the telescope. But Borel wrote nearly fifty years later, after all the principals were dead, and the evidence he collected from the precarious memories of venerable witnesses is very conflicting and points to about 1610 as the date when Jansen was making telescopes—like many other spectacle makers.[1]

Borel also gave credence to a tale that Metius, seeking Jansen, strayed into Lippershey’s shop and by his inquiries gave the shrewd proprietor his first hint of the telescope, but set the date at 1610. A variation of this tale of the mysterious stranger, due to Hieronymus Sirturus, contains the interesting intimation that he may have been of supernatural origin—not further specified. There are also the reports, common among the ignorant or envious, that Lippershey’s discovery was accidental, even perhaps made by his children or apprentice.

Just how it actually was made we do not know, but there is no reason to suppose that it was not in the commonplace way of experimenting with and testing lenses that he had produced, perhaps those made to meet a vicious case of myopia in one of his patrons.

When the discovery was made is somewhat clearer. Plainly it antedated Oct. 2, and in Lippershey’s petition is a definite statement that an instrument had already been tested by some, at least, of the members of the States-General. A somewhat vague and gossipy note in the Mercure Française intimates that one was presented to Prince Maurice “about September of the past year” (1608) and that it was shown to the Council of State and to others.

Allowing a reasonable time between Lippershey’s discovery and the actual production of an example suitable for exhibition to the authorities, it seems likely that the invention dates back certainly into the summer of 1608, perhaps even earlier.

At all events there is every indication that the news of it spread like wild-fire. Unless Lippershey were unusually careful in keeping his secret, and there are traditions that he was not, the sensational discovery would have been quickly known in the little town and every spectacle maker whose ears it reached would have been busy with it.

If the dates given by Simon Marius in his Mundus Jovialis be correct, a Belgian with an air of mystery and a glass of which one of the lenses was cracked, turned up at the Frankfort fair in the autumn of 1608 and at last allowed Fuchs, a nobleman of Bimbach, to look through the instrument. Fuchs noted that it magnified “several” times, but fell out with the Belgian over the price, and returning, took up the matter with Marius, fathomed the construction, tried it with glasses from spectacles, attempted to get a convex lens of longer focus from a Nuremburg maker, who had no suitable tools, and the following summer got a fairly good glass from Belgium where such were already becoming common.

With this Marius eventually picked up three satellites of Jupiter—the fourth awaited the arrival of a superior telescope from Venice. Early in 1609 telescopes “about a foot long” were certainly for sale in Paris, a Frenchman had offered one in Milan by May of that year, a couple of months later one was in use by Harriot in England, an example had reached Cardinal Borghese, and specimens are said to have reached Padua. Fig. 2 from the “Mundus Jovialis,” shows Marius with his “Perspicilium,” the first published picture of the new instrument. Early in 1610 telescopes were being made in England, but if the few reports of performance, even at this date, are trustworthy, the “Dutch trunk” of that period was of very indifferent quality and power, far from being an astronomical instrument.

The Observatory.
Fig. 2.—Simon Marius and his Telescope.

One cannot lay aside this preliminary phase of the evolution of the telescope without reference to the alleged descriptions of telescopic apparatus by Roger Bacon, (c. 1270), Giambattista della Porta (1558), and Leonard Digges (1571), details of which may be found in Grant’s History of Physical Astronomy and many other works.

Of these the first on careful reading conveys strongly the conviction that the author had a pretty clear idea of refraction from the standpoint of visual angle, yet without giving any evidence of practical acquaintance with actual apparatus for doing the things which he suggests.

Given a suitable supply of lenses, it is reasonably certain that Bacon was clever enough to have devised both telescope and microscope, but there is no evidence that he did so, although his manifold activities kept him constantly in public view. It does not seem unlikely, however, that his suggestions in manuscripts, quite available at the time, may have led to the contemporaneous invention of spectacles.

Porta’s comments sound like an echo of Bacon’s, plus a rather muddled attempt to imagine the corresponding apparatus. Kepler, certainly competent and familiar with the principles of the telescope, found his description entirely unintelligible. Porta, however, was one of the earliest workers on the camera obscura and upon this some of his cryptic statements may have borne.

Somewhat similar is the situation respecting Digges. His son makes reference to a Ms. of Roger Bacon as the source of the marvels he describes. The whole account, however, strongly suggests experiments with the camera obscura rather than with the telescope.

The most that can be said with reference to any of the three is that, if he by any chance fell upon the combination of lenses that gave telescopic vision, he failed to set down the facts in any form that could be or was of use to others. There is no reason to believe that the Dutch discovery, important as it was, had gone beyond the empirical observation that a common convex spectacle lens and a concave one of relatively large curvature could be placed in a tube, convex ahead, at such a distance apart as to give a clear enlarged image of distant objects.

It remained for Galileo (1564-1647) to grasp the general principles involved and to apply them to a real instrument of research. It was in May 1609 that, on a visit to Venice, he heard reports that a Belgian had devised an instrument which made distant objects seem near, and this being quickly confirmed by a letter from Paris he awakened to the importance of the issue and, returning to Padua, is said to have solved the problem the very night of his arrival.

Next day he procured a plano-convex and a plano-concave lens, fitted them to a lead tube and found that the combination magnified three diameters, an observation which indicates about what it was possible to obtain from the stock of the contemporary spectacle maker.[2] The relation between the power and the foci of the lenses he evidently quickly fathomed for his next recorded trial reached about eight diameters.

With this instrument he proceeded to Venice and during a month’s stay, August, 1609, exhibited it to the senators of the republic and throngs of notables, finally disclosing the secret of its construction and presenting the tube itself to the Doge sitting in full council. This particular telescope was about twenty inches long and one and five eighths inches in aperture, showing plainly that Galileo had by this time found, or more likely made, an eye lens of short focus, about three inches, quite probably using a well polished convex lens of the ordinary sort as objective.

Lodge “Pioneers of Science.”
Fig. 3.—Galileo.

Laden with honors he returned to Padua and settled down to the hard work of development, grinding many lenses with his own hands and finally producing the instrument magnifying some 32 times, with which he began the notable succession of discoveries that laid the foundation of observational astronomy. This with another of similar dimensions is still preserved at the Galileo Museum in Florence, and is shown in the Frontispiece. The larger instrument is forty-nine inches long and an inch and three quarters aperture, the smaller about thirty-seven inches long and of an inch and five-eighths aperture. The tubes are of paper, the glasses still remain, and these are in fact the first astronomical telescopes.

Galileo made in Padua, and after his return to Florence in the autumn of 1610, many telescopes which found their way over Europe, but quite certainly none of power equalling or exceeding these.

In this connection John Greaves, later Savilian Professor of Astronomy at Oxford, writing from Sienna in 1639, says: “Galileus never made but two good glasses, and those were of old Venice glass.” In these best telescopes, however, the great Florentine had clearly accomplished a most workmanlike feat. He had brought the focus of his eye lens down to that usual in modern opera glasses, and has pushed his power about to the limit for simple lenses thus combined.

The lack of clear and homogeneous glass, the great difficulty of forming true tools, want of suitable commercial abrasives, impossibility of buying sheet metals or tubing (except lead), and default of now familiar methods of centering and testing lenses, made the production of respectably good instruments a task the difficulty of which it is hard now to appreciate.

The services of Galileo to the art were of such profound importance, that his form of instrument may well bear his name, even though his eyes were not the first that had looked through it. Such, too, was the judgment of his contemporaries, and it was by the act of his colleagues in the renowned Acaddemia dei Lincei, through the learned Damiscianus, that the name “Telescope” was devised and has been handed down to us.

A serious fault of the Galilean telescope was its very small field of view when of any considerable power. Galileo’s largest instrument had a field of but 7′15″, less than one quarter the moon’s diameter. The general reason is plain if one follows the rays through the lenses as in Fig. 4 where AB is the distant object, o the objective, e the eye lens, ab the real image in the absence of e, and a′b′ the virtual magnified image due to e.

It will be at once seen that the axes of the pencils of rays from all parts of the object, as shown by the heavy lines, act as if they diverged from the optical center of the objective, but diverging still more by refraction through the concave eye lens e, fall mostly outside the pupil of the observer’s eye. In fact the field is approximately measured by the angle subtended by the pupil from the center of o.

To the credit of the Galilean form may be set down the convenient erect image, a sharp, if small, field somewhat bettered by a partial compensation of the aberrations of the objective by the concave eye lens, and good illumination. For a distant object the lenses were spaced at the difference of their focal lengths, and the magnifying power was the ratio of these, f_o/f_e.

Fig. 4.—Diagram of Galileo’s Telescope.

But the difficulty of obtaining high power with a fairly sizeable field was ultimately fatal and the type now survives only in the form of opera and field glasses, usually of 2 to 5 power, and in an occasional negative eye lens for erecting the image in observatory work. Practically all the modern instruments have achromatic objectives and commonly achromatic oculars.

Fig. 5.—Diagram of Kepler’s Telescope.

The necessary step forward was made by Johann Kepler (1571-1630), the immortal discoverer of the laws of planetary motion. In his Dioptrice (1611) he set forth the astronomical telescope, substantially, save for the changes brought by achromatism, as it has been used ever since. His arrangement was that of Fig. 5 in which the letters have the same significance as in Fig. 4.

There are here three striking differences from the Galilean form. There is a real image in the front focus of the eye lens e, the rays passing it are refracted inwards instead of outwards, to the great advantage of the field, and any object placed in the image plane will be magnified together with the image. The first two points Kepler fully realized, the third he probably did not, though it is the basis of the micrometer. The lenses o and e are obviously spaced at the sum of their focal lengths, and as before the magnifying power is the ratio of these lengths, the visible image being inverted.

Kepler, so far as known, did not actually use the new telescope, that honor falling about half a dozen years later, to Christopher Scheiner, a Jesuit professor of mathematics at Ingolstadt, best known as a very early and most persistent, not to say verbose, observer of sun spots. His Rosa Ursina (1630) indicates free use of Kepler’s telescope for some years previously, in just what size and power is uncertain.[3] Fontana of Naples also appears to have been early in the field.

But the new instrument despite its much larger field and far greater possibilities of power, brought with it some very serious problems. With increased power came greatly aggravated trouble from spherical aberration and chromatic aberration as well, and the additive aberrations of the eye lens made matters still worse. The earlier Keplerian instruments were probably rather bad if the drawings of Fontana from 1629 to 1636 fairly represent them.

If one may judge from the course of developments, the first great impulse to improvement came with the publication of Descartes’ (1596-1650) study of dioptrics in 1637. Therein was set forth much of the theory of spherical aberration and astronomers promptly followed the clues, practical and impractical, thus disclosed.

Without going into the theory of aberrations the fact of importance to the improvement of the early telescope is that the longitudinal spherical aberration of any simple lens is directly proportional to its thickness due to curvature. Hence, other things being equal, the longer the focus for the same aperture the less the spherical aberration both absolutely and relatively to the image. Further, although Descartes knew nothing of chromatic aberration, and the colored fringe about objects seen through the telescope must then have seemed altogether mysterious, it, also, was greatly relieved by lengthening the focus.

For the chromatic circle produced by a simple lens of given diameter has a radial width substantially irrespective of the focal length. But increasing the focal length increases in exact proportion the size of the image, correspondingly decreasing the relative effect of the chromatic error.

Descartes also suggested several designs of lenses which would be altogether free of spherical aberration, formed with elliptical or hyperbolic curvature, and for some time fruitless efforts were made to realize this in practice. It was in fact to be near a century before anyone successfully figured non-spherical surfaces. It was spherical quite as much as chromatic aberration that drove astronomers to long telescopes.

Meanwhile the astronomical telescope fell into better hands than those of Scheiner. The first fully to grasp its possibilities was William Gascoigne, a gallant young gentleman of Middleton, Yorkshire, born about 1620 (some say as early as 1612) and who died fighting on the King’s side at Marston Moor, July 2, 1644. To him came as early as 1638 the inspiration of utilizing the real focus of the objective for establishing a telescopic sight.

Fig. 6.—Diagram of Terrestrial Ocular.

This shortly took the form of a genuine micrometer consisting of a pair of parallel blades in the focus, moved in opposite directions by a screw of duplex pitch, with a scale for whole revolutions, and a head divided into 100 parts for partial revolutions. With this he observed much from 1638 to 1643, measured the diameters of sun, moon and planets with a good degree of precision, and laid the foundations of modern micrometry. He was equipped by 1639 with what was then called a large telescope.

His untimely death, leaving behind an unpublished treatise on optics, was a grave loss to science, the more since the manuscript could not be found, and, swept away by the storms of war, his brilliant work dropped out of sight for above a score of years.

Meanwhile De Rheita (1597-1660), a Capuchin monk, and an industrious and capable investigator, had been busy with the telescope, and in 1645 published at Antwerp a somewhat bizarre treatise, dedicated to Jesus Christ, and containing not a little practical information. De Rheita had early constructed binoculars, probably quite independently, had lately been diligently experimenting with Descartes’ hyperbolic lens, it is needless to say without much success, and was meditating work on a colossal scale—a glass to magnify 4,000 times.

But his real contribution to optics was the terrestrial ocular. This as he made it is shown in Fig. 6 where a b is the image formed by the objective in front of the eye lens r, s and t two equal lenses separated by their focal lengths and a′ b′ the resultant reinverted image. This form remained in common use until improved by Dolland more than a century later.

Fig. 7.—Johannes Hevelius.

A somewhat earlier form ascribed to Father Scheiner had merged the two lenses forming the inverting system of Fig. 6, into a single lens used at its conjugate foci.

Closely following De Rheita came Johannes Hevelius (1611-1687) of Danzig, one of the really important observers of the seventeenth century. His great treatise Selenographia published in 1647 gives us the first systematic study of the moon, and a brief but illuminating account of the instruments of the time and their practical construction.

At this time the Galilean and Keplerian forms of telescope were in concurrent use and Hevelius gives directions for designing and making both of them. Apparently the current instruments were not generally above five or six feet long and from Hevelius’ data would give not above 30 diameters in the Galilean form. There is mention, however, of tubes up to 12 feet in length, and of the advantage in clearness and power of the longer focus plano-convex lens. Paper tubes, evidently common, are condemned, also those of sheet iron on account of their weight, and wood was to be preferred for the longer tubes.

Evidently Hevelius had at this time no notion of the effect of the plano-convex form of lens as such in lessening aberration, but he mentions a curious form of telescope, actually due to De Rheita, in which the objective is double, apparently of two plano-convex lenses, the weaker ahead, and used with a concave eye lens. If properly proportioned such a doublet would have less than a quarter the spherical aberration of the equivalent double convex lens.

Hevelius also mentions the earlier form of re-inverting telescope above referred to, and speaks rather highly of its performance. To judge from his numerous drawings of the moon made in 1643 and 1644, his telescopes were much better than those of Scheiner and Fontana, but still woefully lacking in sharp definition.

Nevertheless the copper plates of the Selenographia, representing every phase of the moon, placed the lunar details with remarkable accuracy and formed for more than a century the best lunar atlas available. One acquires an abiding respect for the patience and skill of these old astronomers in seeing how much they did with means utterly inadequate.

One may get a fair idea of the size, appearance, and mounting of telescopes in this early day from Fig. 8, which shows a somewhat advanced construction credited by Hevelius to a suggestion in Descartes’ Dioptrica. Appearances indicate that the tube was somewhere about six feet long, approximately two inches in aperture, and that it had a draw tube for focussing. The offset head of the mount to allow observing near the zenith is worth an extra glance.

Incidentally Hevelius, with perhaps pardonable pride, also explains the “Polemoscope,” a little invention of his own, made, he tells us, in 1637. It is nothing else than the first periscope, constructed as shown in Fig. 9, a tube c with two right angled branches, a fairly long one e for the objective f, a 45° mirror at g, another at a, and finally the concave ocular at b. It was of modest size, of tubes 1⅔ inch in diameter, the longer tube being 22 inches and the upper branch 8 inches, a size well suited for trench or parapet.

Fig. 8.—A Seventh Century Astronomer and his Telescope.

Even in these days of his youth Hevelius had learned much of practical optics as then known, had devised and was using very rational methods of observing sun-spots by projection in a darkened room, and gives perhaps the first useful hints at testing telescopes by such solar observations and on the planets. He was later to do much in the development and mounting of long telescopes and in observation, although, while progressive in other respects, he very curiously never seemed to grasp the importance of telescopic sights and consistently refused to use them.

Telescope construction was now to fall into more skillful hands. Shortly after 1650 Christian Huygens (1629-1695), and his accomplished brother Constantine awakened to a keen interest in astronomy and devised new and excellent methods of forming accurate tools and of grinding and polishing lenses.

Fig. 9.—The first Periscope.

By 1655 they had completed an instrument of 12 feet focus with which the study of Saturn was begun, Titan the chief satellite discovered, and the ring recognized. Pushing further, they constructed a telescope of 23 feet focal length and 2⅓ inches aperture, with which four years later Christian Huygens finally solved the mystery of Saturn’s ring.

Evidently this glass, which bore a power of 100, was of good defining quality, as attested by a sketch of Mars late in 1695 showing plainly Syrtis Major, from observation of which Huygens determined the rotation period to be about 24 hours.

The Huygens brothers were seemingly the first fully to grasp the advantage of very long focus in cutting down the aberrations, the aperture being kept moderate. Their usual proportions were about as indicated above, the aperture being kept somewhere nearly as the square root of the focus in case of the larger glasses.

In the next two decades the focal length of telescopes was pushed by all hands to desperate extremes. The Huygens brothers extended themselves to glasses up to 210 feet focus and built many shorter ones, a famous example of which, of 6 inches aperture and 123 feet focal length, presented to the Royal Society, is still in its possession. Auzout produced even longer telescopes, and Divini and Campani, in Rome, of whom the last named made Cassini’s telescopes for the Observatory of Paris, were not far behind. The English makers were similarly busy, and Hevelius in Danzig was keeping up the record.

Fig. 10.—Christian Huygens.

Clearly these enormously long telescopes could not well be mounted in tubes and the users were driven to aerial mountings, in which the objective was at the upper end of a spar or girder and the eye piece at the lower. Figure 11 shows an actual construction by Hevelius for an objective of 150 feet focal length.

In this case the main support was a T beam of wooden planks well braced together. Additional stiffness was given by light wooden diaphragms at short intervals with apertures of about 8 inches next to the objective, and gradually increasing downwards. The whole was lined up by equalizing tackle in the vertical plane, and spreaders with other tackle at the joints of the 40foot sections of the main beam. The mast which supported the whole was nearly 90 feet high.

So unwieldly and inconvenient were these long affairs that, quite apart from their usual optical imperfections, it is little wonder that they led to no results commensurate with their size. In fact nearly all the productive work was done with telescopes from 20 to 35 feet long, with apertures roughly between 2 and 3 inches.

Fig. 11.—Hevelius’ 150-foot Telescope.

Dominique Cassini to be sure, scrutinizing Saturn in 1684 with objectives by Campani, of 100 and 136 feet focus picked up the satellites Tethys and Dione, but he had previously found Iapetus with a 17-foot glass, and Rhea with one of 34 feet. The longer glasses above mentioned had aerial mounts but the smaller ones were in tubes supported on a sort of ladder tripod. A 20-foot objective, power 90, gave Cassini the division in Saturn’s ring.

A struggle was still being kept up for the non-spherical curves urged by Descartes. It is quite evident that Huygens had a go at them, and Hevelius thought at one time that he had mastered the hyperbolic figure, but his published drawings give no indication that he had reduced spherical aberration to any perceptible degree. At this time the main thing was to get good glass and give it true figure and polish, in which Huygens and Campani excelled, as the work on Saturn witnesses.

These were the days of the dawn of popular astronomy and many a gentleman was aroused to at least a casual interest in observing the Heavens. Notes Pepys in his immortal Diary: “I find Reeves there, it being a mighty fine bright night, and so upon my leads, though very sleepy, till one in the morning, looking on the moon and Jupiter, with this twelve foot glass, and another of six foot, that he hath brought with him to-night, and the sights mighty pleasant, and one of the glasses I will buy.”

Little poor Pepys probably saw, by reason of his severe astigmatism, but astronomy was in the air with the impulse that comes to every science after a period of brilliant discovery. Another such stimulus came near the end of the eighteenth century, with the labors of Sir William Herschel.

Fig. 12.—Gregory’s Diagram of his Telescope.

Just at this juncture comes one of the interesting episodes of telescopic history, the ineffectual and abandoned experiments on reflecting instruments.

In 1663 James Gregory (1638-1675) a famous Scottish mathematician, published his Optica Promota, in which he described the rather elegant construction which bears his name, a perforated parabolic mirror with an elliptical mirror forward of the focus returning an image to the ocular through the perforation. It was convenient in that it gave an erect image, and it was sound theoretically, and, as the future proved, practically, but the curves were quite too much for the contemporary opticians. Figure 12 shows the diagrammatic construction as published.

The next year Gregory started Reive, a London optician, doubtless the same mentioned by Pepys, on the construction of a 6 foot telescope. This rather ambitious effort failed of material success through the inability of Reive to give the needed figures to the mirrors,[4] and of it nothing further appears until the ingenious Robert Hooke (1635-1703) executed in 1674 a Gregorian, apparently without any notable results. There is a well defined tradition that Gregory himself was using one in 1675, at the time of his death, but the invention then dropped out of sight.

No greater influence on the art attended the next attempt at a reflector, by Isaac Newton (1643-1727). This was an early outcome of his notable discovery of the dispersion of light by prisms, which led him to despair of improving refracting telescopes and turned his mind to reflectors.

Unhappily in an experiment to determine whether refraction and dispersion were proportional he committed the singular blunder of raising the refractive index of a water-filled prism to equality with glass by dissolving sugar of lead in it. Without realizing the impropriety of thus varying two quite unknown quantities at once in his crucial experiment, he promptly jumped to the conclusion that refraction and dispersion varied in exact proportion in all substances, so that if two prisms or lenses dispersed light to the same extent they must also equally refract it. It would be interesting to know just how the fact of his bungling was passed along to posterity. As a naïve apologist once remarked, it was not to be found in his “Optics.” But Sir David Brewster and Sir John Herschel, both staunch admirers of the great philosopher, state the fact very positively. If one may hazard a guess it crept out at Cambridge and was passed along, perhaps to Sir William Herschel, via the unpublished history of research that is rich in picturesque details of the mare’s nests of science. At all events a mistake with a great name behind it carries far, and the result was to delay the production of the achromatic telescope by some three quarters of a century.

Turning from refractors he presented to the Royal Society just after his election as Fellow in 1672, the little six-inch model of his device which was received with acclamation and then lay on the shelf without making the slightest impression on the art, for full half a century.

Newton, by dropping the notion of direct view through the tube, hit upon by far the simplest way of getting the image outside it, by a plane mirror a little inside focus and inclined at 45°, but injudiciously abandoned the parabolic mirror of his original paper on dispersion. His invention therefore as actually made public was of the combination with a spherical concave mirror of a plane mirror of elliptical form at 45°, a construction which in later papers he defended as fully adequate.[5]

Fig. 13.—Newton’s Model of his Reflector.

His error in judgment doubtless came from lack of practical astronomical experience, for he assumed that the whole real trouble with existing telescopes was chromatic aberration, which in fact worried the observer little more than the faults due to other causes, since the very low luminosity toward the ends of the spectrum enormously lessens the indistinctness due to dispersion.

As a matter of fact the long focus objective of small aperture did very creditable work, and its errors would not compare unfavorably with those of a spherical concave mirror of the wide aperture planned by Newton. Had he actually made one of his telescopes of fair dimensions and power the definition would infallibly have been wrecked by the aberrations due to spherical figure.[6]

Fig. 14.—De Bercé’s sketch of Cassegrain’s Telescope.

It is quite likely that appreciation of this, and the grave doubts of both Newton and Huygens as to obtaining a proper parabolic curve checked further developments. About the beginning of the year 1672 M. Cassegrain communicated to M. de Bercé a design for a reflecting telescope, which eventually found its way into the Philosophical Transactions of May in that year, after previous publication in the Journal des Sçavans. Figure 14 shows de Bercé’s rough original sketch. It differed from Gregory’s construction in that the latter’s elliptical concave mirror placed outside the main focus, was replaced by a convex mirror placed inside focus. The image was therefore inverted.

The inventor is referred to in histories of science as “Cassegrain, a Frenchman.” He was in fact Sieur Guillaume Cassegrain, sculptor in the service of Louis Quatorze, modeller and founder of many statues. In 1666 he was paid 1200 livres for executing a bust of the King modelled by Bertin, and later made many replicas from the antique for the decoration of His Majesty’s gardens at Versailles. He disappeared from the royal records in 1684 and probably died within a year or two of that date.

At the period here concerned he apparently, like de Bercé, was of Chartres. Familiar with working bronzes and with the art of the founder, he was a very likely person to have executed specula. Although there is no certainty that he actually made a telescope, a contemporary reference in the Journal des Sçavans speaks of his invention as a “petite lunette d’approche,” and one does not usually suggest the dimensions of a thing non-existent. How long he had been working upon it prior to the period about the beginning of 1672 when he disclosed the device to de Bercé is unknown.

Probably Newton’s invention was the earlier, but the two were independent, and it was somewhat ungenerous of Newton to criticise Cassegrain, as he did, for using spherical mirrors, on the strength of de Bercé’s very superficial description, when he himself considered the parabolic needless.

However, nothing further was done, and the devices of Gregory, Newton and Cassegrain went together into the discard for some fifty years.

These early experiments gave singularly little information about material for mirrors and methods of working it, so little that those who followed, even up to Lord Rosse, had to work the problems out for themselves. We know from his original paper that Newton used bell-metal, whitened by the addition of arsenic, following the lore of the alchemists.

These speculative worthies used to alloy copper with arsenic, thinking that by giving it a whitish cast they had reached a sort of half way point on the road to silver. Very silly at first thought, but before the days of chemical analysis, when the essential properties of the metals were unknown, the way of the scientific experimenter was hard.

What the “steely matter, imployed in London” of which Newton speaks in an early paper was, we do not know—very likely one of the hard alloys much richer in tin than is ordinary bell-metal. Nor do we know to what variety of speculum metal Huygens refers in his correspondence with Newton.

As to methods of working it Newton only disclosed his scheme of pitch-polishing some thirty years after this period, while it is a matter of previous record, that Huygens had been in the habit of polishing his true tools on pitch from some date unknown. Probably neither of them originated the practice. Opticians are a peculiarly secretive folk and shop methods are likely to be kept for a long time before they leak out or are rediscovered.

Modern speculum metal is substantially a definite compound of four atoms copper and one tin (SnCu₄), practically 68 per cent copper and 32 per cent tin, and is now, as it was in all previous modifications, a peculiarly mean material to cast and work. Thus exit the reflector.

Fig. 15.—Diagram of Huygens’ Eyepiece.

The long telescope continued to grow longer with only slow improvement in quality, but the next decade was marked by the introduction of Huygens’ eyepiece, an immense improvement over the single lens which had gone before, and with slight modifications in use today.

This is shown in section in Fig. 15. It consists of a field lens A, plano-convex, and an eye lens B of one-third the focal length, the two being placed at the difference of their focal lengths apart with (in later days) a stop half way between them. The eye piece is pushed inside the main focus until the rays which fall on the field lens focus through the eye lens.

The great gain from Huygens’ view-point was a very much enlarged clear field—about a four-fold increase—and in fact the combination is substantially achromatic, particularly important now when high power oculars are needed.

Still larger progress was made in giving the objective a better form with respect to spherical aberration, the “crossed” lens being rather generally adopted. This form is double convex, and if of ordinary glass, with the rear radius six times the front radius, and gives even better results than a plano-convex in its best position-plane side to the rear. Objectives were rated on focal length for the green rays, that is, the bright central part of the spectrum, the violet rays of course falling short and the red running beyond.

To give customary dimensions, a telescope of 3 inches aperture, with magnifying power of 100, would be of about 30 feet focus with the violet nearly 6 inches short and the red a similar amount long. It is vast credit to the early observers that with such slender means they did so much. But in fact the long telescope had reached a mechanical impasse, so that the last quarter of the seventeenth century and the first quarter of the next were marked chiefly by the development of astronomy of position with instruments of modest dimensions.

Fig. 16.—The First Reflector. John Hadley, 1722.

In due time the new order came and with astounding suddenness. Just at the end of 1722 James Bradley (1692-1762) measured the diameter of Venus with an objective of 212 ft. 3 in. focal length; about three months later John Hadley (1682-1744) presented to the Royal Society the first reflecting telescope worthy the name, and the old order practically ended.

John Hadley should in fact be regarded as the real inventor of the reflector in quite the same sense that Mr. Edison has been held, de jure and de facto, the inventor of the incandescent electric lamp. Actually Hadley’s case is the stronger of the two, for the only things which could have been cited against him were abandoned experiments fifty years old. Moreover he took successfully the essential step at which Gregory and Newton had stumbled or turned back—parabolizing his speculum.

The instrument he presented was of approximately 6 inches aperture and 62⅝ inches focal length, which he had made and tested some three years previously; on a substantial alt-azimuth mount with slow motions. He used the Newtonian oblique mirror and the instrument was provided with both convex and concave eye lenses, with magnifications up to about 230.

The whole arrangement is shown in Fig. 16 which is for the most part self explanatory. It is worth noting that the speculum is positioned in the wooden tube by pressing it forward against three equidistant studs by three corresponding screws at the rear, that a slider moved by a traversing screw in a wide groove carries the small mirror and the ocular, that there is a convenient door for access to the mirror, and also a suitable finder. The motion in altitude is obtained by a key winding its cord against gravity. That in azimuth is by a roller support along a horizontal runway carried by an upright, and is obtained by the key with a cord pull off in one direction, and in the other, by springs within the main upright, turning a post of which the head carries cheek pieces on which rest the trunnions of the tube.

A few months later this telescope was carefully tested, by Bradley and the Rev. J. Pound, against the Huygens objective of 123 feet focus possessed by the Royal Society, and with altogether satisfactory results. Hadley’s reflector would show everything which could be seen by the long instrument, bearing as much power and with equal definition, though somewhat lessened light. In particular they saw all five satellites of Saturn, Cassini’s division, which the inventor himself had seen the previous year even in the northern edge of the ring beyond the planet, and the shadow of the ring upon the ball.

The casting of the large speculum was far from perfect, with many spots that failed to take polish, but the figure must have been rather good. A spherical mirror of these dimensions would give an aberration blur something like twenty times the width of Cassini’s division, and the chance of seeing all five satellites with it would be negligibly small.

Further, Hadley presently disclosed to others not only the method he used in polishing and parabolizing specula, but his method of testing for true figure by the aberrations disclosed as he worked the figure away from the sphere—a scheme frequently used even to this day.

The effect of Hadley’s work was profound. Under his guidance others began to produce well figured mirrors, in particular Molyneux and Hawksbee; reflecting telescopes became fairly common; and in the beginning of the next decade James Short, (1710-1768), possessed of craftsmanship that approached wizardry, not only fully mastered the art of figuring the paraboloid, but at once took up the Gregorian construction with its ellipsoidal small mirror, with much success.

His specula were of great relative aperture, F/4 to F/6, and from the excellent quality of his metal some of them have retained their fine polish and definition after more than a century. He is said to have gone even up to 12 inches in diameter. His exact methods of working died with him. Even his tools he ordered to be destroyed before his death.

The Cassegrain reflector, properly having a parabolic large mirror and a hyperbolic small one, seems very rarely to have been made in the eighteenth century, though one certainly came into the hands of Ramsden (1735-1800).

Few refractors for astronomical use were made after the advent of the reflector, which was, and is, however, badly suited for the purposes of a portable spy-glass, owing to trouble from stray light. The refractor therefore permanently held its own in this function, despite its length and uncorrected aberrations.

Relief was near at hand, for hardly had Short started on his notable career when Chester Moor Hall, Esq. (1704-1771) a gentleman of Essex, designed and caused to be constructed the first achromatic telescope, with an objective of crown and flint glass. He is stated to have been studying the problem for several years, led to it by the erroneous belief (shared by Gregory long before) that the human eye was an example of an achromatic instrument.

Be this as it may Hall had his telescopes made by George Bast of London at least as early as 1733, and according to the best available evidence several instruments were produced, one of them of above 2 inches aperture on a focal length of about 20 inches (F/8) and further, subsequently such instruments were made and sold by Bast and other opticians.

These facts are clear and yet, with knowledge of them among London workmen as well as among Hall’s friends, the invention made no impression, until it was again brought to light, and patented, by the celebrated John Dolland (1706-1761) in the year 1758.

Physical considerations give a clue to this singular neglect. The only glasses differing materially in dispersion available in Hall’s day were the ordinary crown, and such flint as was in use in the glass cutting trade,—what we would now know as a light flint, and far from homogeneous at that.

Lodge “Pioneers of Science.”
Fig. 17.—John Dolland.

Out of such material it was practically very hard (as the Dollands quickly found) to make a double objective decently free from spherical aberration, especially for one working, as Hall quite assuredly did, by rule of thumb. With the additional handicap of flint full of faults it is altogether likely that these first achromatics, while embodying the correct principles, were not good enough to make effective headway against the cheaper and simpler spy-glass of the time.

Dolland, although in 1753 he strongly supported Newton’s error in a Royal Society paper against Euler’s belief in achromatism, shifted his view a couple of years later and after a considerable period of skilful and well ordered experimenting published his discovery of achromatism early in 1758, for which a patent was granted him April 19, while in the same year the Royal Society honored him with the Copley medal. From that time until his death, late in 1761, he and his son Peter Dolland (1730-1820) were actively producing achromatic glasses.

The Dollands were admirable craftsmen and their early product was probably considerably better than were Hall’s objectives but they felt the lack of suitable flint and soon after John Dolland’s death, about 1765, the son sought relief in the triple objective of which an early example is shown in Fig. 18, and which, with some modifications, was his standard form for many years.

Fig. 18.—Peter Dolland’s Triple Objective.

Other opticians began to make achromatics, and, Peter Dolland having threatened action for infringement, a petition was brought by 35 opticians of London in 1764 for the annulment of John Dolland’s patent, alleging that he was not the original inventor but had knowledge of Chester Moor Hall’s prior work. In the list was George Bast, who in fact did make Hall’s objectives twenty five years before Dolland, and also one Robert Rew of Coldbath Fields, who claimed in 1755 to have informed Dolland of the construction of Hall’s objective.

This was just the time when Dolland came to the right about face on achromatism, and it may well be that from Rew or elsewhere he may have learned that a duplex achromatic lens had really been produced. But his Royal Society paper shows that his result came from honest investigations, and at worst he is in about the position of Galileo a century and a half before.

The petition apparently brought no action, perhaps because Peter Dolland next year sued Champneys, one of the signers, and obtained judgment. It was in this case that the judge (Lord Camden) delivered the oft quoted dictum: “It was not the person who locked up his invention in his scrutoire that ought to profit by a patent for such invention, but he who brought it forth for the benefit of the public.[7]”

This was sound equity enough, assuming the facts to be as stated, but while Hall did not publish the invention admittedly made by him, it had certainly become known to many. Chester Moor Hall was a substantial and respected lawyer, a bencher of the Inner Temple, and one is inclined to think that his alleged concealment was purely constructive, in his failing to contest Dolland’s claim.

Had he appeared at the trial with his fighting blood up, there is every reason to believe that he could have established a perfectly good case of public use quite aside from his proof of technical priority. However, having clearly lost his own claims through laches, he not improbably was quite content to let the tradesmen fight it out among themselves. Hall’s telescopes were in fact known to be in existence as late as 1827.

As the eighteenth century drew toward its ending the reflecting telescope, chiefly in the Gregorian form, held the field in astronomical work, the old refractor of many draw tubes was the spy-glass of popular use, and the newly introduced achromatic was the instrument of “the exclusive trade.” No glass of suitable quality for well corrected objectives had been produced, and that available was not to be had in discs large enough for serious work. A 3-inch objective was reckoned rather large.

[CHAPTER II]
THE MODERN TELESCOPE

The chief link between the old and the new, in instrumental as well as observational astronomy, was Sir William Herschel (1738-1822). In the first place he carried the figuring of his mirrors to a point not approached by his predecessors, and second, he taught by example the immense value of aperture in definition and grasp of light. His life has never been adequately written, but Miss Clerke’s “The Herschels and Modern Astronomy” is extremely well worth the reading as a record of achievement that knew not the impossible.

Miss Clerke’s Herschel & Modern Astronomy (Macmillan).
Fig. 19.—Sir William Herschel.

He was the son of a capable band-master of Hanover, brought up as a musician, in a family of exceptional musical abilities, and in 1757 jumped his military responsibilities and emigrated to England, to the world’s great gain. For nearly a decade he struggled upward in his art, taking meanwhile every opportunity for self education, not only in the theory of music but in mathematics and the languages, and in 1767 we find him settled in fashionable Bath, oboist in a famous orchestra, and organist of the Octagon Chapel. His abilities brought him many pupils, and ultimately he became director of the orchestra in which he had played, and the musical dictator of the famous old resort.

In 1772 came his inspiration in the loan of a 2-foot Gregorian reflector, and a little casual star-gazing with it. It was the opening of the kingdom of the skies, and he sought to purchase a telescope of his own in London, only to find the price too great for his means. (Even a 2-foot, of 4½ inches aperture, by Short was listed at five-and-thirty guineas.) Then after some futile attempts at making a plain refractor he settled down to hard work at casting and polishing specula.

Although possessed of great mechanical abilities the difficult technique of the new art long baffled him, and he cast and worked some 200 small discs in the production of his first successful telescopes, to say nothing of a still greater number in larger sizes in his immediately subsequent career.

As time went on he scored a larger proportion of successes, but at the start good figure seems to have been largely fortuitous. Inside of a couple of years, however, he had mastered something of the art and turned out a 5-foot instrument which seems to have been of excellent quality, followed later by a 7-foot (aperture 6¼ inches) even better, and then by others still bigger.

The best of Herschel’s specula must have been of exquisite figure. His 7-foot was tested at Greenwich against one of Short’s of 9½ inches aperture much to the latter’s disadvantage. His discovery with the 7-foot, of the “Georgium Sidus” (Uranus) in 1781 won him immediate fame and recognition, beside spurring him to greater efforts, especially in the direction of larger apertures, of which he had fully grasped the importance.

In 1782 he successfully completed a 12-inch speculum of 20 feet focus, followed in 1788 by an 18-inch of the same length. The previous year he first arranged his reflector as a “front view” telescope—the so-called Herschelian. Up to this time he, except for a few Gregorians, had used Newton’s oblique mirror.

The heavy loss of light (around 40 per cent) in the second reflection moved him to tilt the main mirror so as to throw the focal point to the edge of the aperture where one could look downward upon the image through the ocular as shown in Fig. 20. Here SS is the great speculum, O the ocular and i the image formed near the rim of the tube. In itself the tilting would seriously impair the definition, but Herschel wisely built his telescopes of moderate relative aperture (F/10 to F/20), so that this difficulty was considerably lessened, while the saving of light, amounting to nearly a stellar magnitude, was important.

Meanwhile he was hard at work on his greatest mirror, of 48 inches clear aperture and 40 feet focal length, the father of the great line of modern telescopes. It was finished in the summer of 1789. The speculum was 49½ inches in over-all diameter, 3½ inches thick and weighed as cast 2118 lbs. The completion of this instrument, which would rank as large even today, was made notable by the immediate discovery of two new satellites of Saturn, Enceladus and Mimas.

It also proved of very great value in sweeping for nebulæ, but its usefulness seems to have been much limited by the flexure of the mirror under its great weight, and by its rapid tarnishing. It required repolishing, which meant refiguring, at least every two years, a prodigious task.[8]

Fig. 20.—Herschel’s Front View Telescope.

It was used as a front view instrument and was arranged as shown in Fig. 21. Obviously the front view form has against it the mechanical difficulty of supporting the observer up to quite the full focal length of the instrument in air, a difficulty vastly increased were the mount an equatorial one, so that for the great modern reflectors the Cassegrain form, looked into axially upward, and in length only a third or a quarter of the principal focus, is almost universal.

As soon as the excellent results obtained by Herschel became generally known, a large demand arose for his telescopes, which he filled in so far as he could spare the time from his regular work, and not the least of his services to science was the distribution of telescopes of high quality and consequent strong stimulus to general interest in astronomy.

Two of his instruments, of 4-and 7-feet focus respectively, fell into the worthy hands of Schröter at Lilienthal and did sterling service in making his great systematic study of the lunar surface. At the start even Herschel’s 7-foot telescope brought 200 guineas, and the funds thus won he promptly turned to research.

Miss Clerke’s Herschel & Modern Astronomy (Macmillan).
Fig. 21.—Herschel’s Forty-foot Telescope.

We sometimes think of the late eighteenth century as a time of license unbounded and the higher life contemned, but Herschel wakened a general interest in unapplied science that has hardly since been equalled and never surpassed. Try to picture social and official Washington rushing to do honor to some astronomer who by luck had found the trans-Neptunian planet; the diplomatic corps crowding his doors, and his very way to the Naval Observatory blocked by the limousines of the curious and admiring, and some idea may be gained of what really happened to the unassuming music master from Bath who suddenly found himself famous.

Great as were the advances made by Herschel the reflector was destined to fall into disuse for many years. The fact was that the specula had to be refigured, as in the case of the great 40-foot telescope, quite too often to meet the requirements of the ordinary user, professional or amateur. Only those capable of doing their own figuring could keep their instruments conveniently in service.

Sir W. Herschel always had relays of specula at hand for his smaller instruments, and when his distinguished son, Sir John F. W. Herschel, went on his famous observing expedition to the Cape of Good Hope in 1834-38 he took along his polishing machine and three specula for his 20-foot telescope. And he needed them indeed, for a surface would sometimes go bad even in a week, and regularly became quite useless in 2 or 3 months.

Makers who used the harder speculum metal, very brittle and scarcely to be touched by a file, fared better, and some small mirrors, well cared for, have held serviceable polish for many years. Many of these instruments of Herschel’s time, too, were of very admirable performance.

Some of Herschel’s own 7-foot telescopes give evidence of exquisite figure and he not only commonly used magnifying powers up to some 80 per inch of aperture, a good stiff figure for a telescope old or new, but went above 2,000, even nearly to 6,000 on one of his 6½-inch mirrors without losing the roundness of the star image. “Empty magnification” of course, gaining no detail whatever, but evidence of good workmanship.

Many years later the Rev. W. R. Dawes, the famous English observer, had a 5-inch Gregorian, commonly referred to as “The Jewel,” on which he used 430 diameters, and pushed to 2,000 on Polaris without distortion of the disc. Comparing it with a 5-foot (approximately 4-inch aperture) refractor, he reports the Gregorian somewhat inferior in illuminating power; “But in sharpness of definition, smallness of discs of stars, and hardness of outline of planets it is superior.” All of which shows that while methods and material may have improved, the elders did not in the least lack skill.

The next step forward, and a momentous one, was to be taken in the achromatic refractor. Its general principles were understood, but clear and homogeneous glass, particularly flint glass, was not to be had in pieces of any size. “Optical glass,” as we understand the term, was unknown.

It is a curious and dramatic fact that to a single man was due not only the origin of the art but the optical glass industry of the world. If the capacity for taking infinite pains be genius, then the term rightfully belongs to Pierre Louis Guinand. He was a Swiss artisan living in the Canton of Neuchatel near Chaux-de-Fonds, maker of bells for repeaters, and becoming interested in constructing telescopes imported some flint glass from England and found it bad.

He thereupon undertook the task of making better, and from 1784 kept steadily at his experiments, failure only spurring him on to redoubled efforts. All he could earn at his trade went into his furnaces, until gradually he won success, and his glass began to be heard of; for by 1799 he was producing flawless discs of flint as much as 6 inches in diameter.

What is more, to Guinand is probably due the production of the denser, more highly refractive flints, especially valuable for achromatic telescopes. The making of optical glass has always been an art rather than a science. It is one thing to know the exact composition of a glass and quite another to know in what order and proportion the ingredients went into the furnace, to what temperature they were carried, and for how long, and just how the fused mass must be treated to free the products from bubbles and striæ.

Even today, though much has been learned by scientific investigation in the past few years, it is far from easy to produce two consecutive meltings near enough in refractive power to be treated as optically identical, or to produce large discs optically homogeneous. What Guinand won by sheer experience was invaluable. He was persuaded in 1805 to move to Munich and eventually to join forces with Fraunhofer, an association which made both the German optical glass industry and the modern refractor.

He returned to Switzerland in 1814 and continued to produce perfect discs of larger and larger dimensions. One set of 12 inches worked up by Cauchoix in Paris furnished what was for some years the world’s largest refractor.

Guinaud died in 1824, but his son Henry, moving to Paris, brought his treasure of practical knowledge to the glass works there, where it has been handed down, in effect from father to son, gaining steadily by accretion, through successive firms to the present one of Parra-Mantois.

Bontemps, one of the early pupils of Henry Guinand, emigrated to England at the Revolution of 1848 and brought the art to the famous firm of Chance in Birmingham. Most of its early secrets have long been open, but the minute teachings of experience are a tremendously valuable asset even now.

Fig. 22.—Dr. Joseph von Fraunhofer, the Father of Astrophysics.

To Fraunhofer, the greatest master of applied optics in the nineteenth century, is due the astronomical telescope in substantially its present form. Not only did he become under Guinand’s instruction extraordinarily skillful in glass making but he practically devised the art of working it with mathematical precision on an automatic machine, and the science of correctly designing achromatic objectives.

The form which he originated (Fig. 23) was the first in which the aberrations were treated with adequate completeness, and, particularly for small instruments, is unexcelled even now. The curvatures here shown are extreme, the better to show their relations. The front radius of the crown is about 2½ times longer than the rear radius, the front of the flint is slightly flatter than the back of the crown, and the rear of the flint is only slightly convex.

Fraunhofer’s workmanship was of the utmost exactness and it is not putting the case too strongly to say that a first class example of the master’s craft, in good condition, would compare well in color-correction, definition, and field, with the best modern instruments.

Fig. 23.

The work done by the elder Struve at Dorpat with Fraunhofer’s first large telescope (9.6 inches aperture and 170 inches focal length) tells the story of its quality, and the Königsberg heliometer, the first of its class, likewise, while even today some of his smaller instruments are still doing good service.

It was he who put in practice the now general convention of a relative aperture of about F/15, and standardized the terrestrial eyepiece into the design quite widely used today. The improvements since his time have been relatively slight, due mainly to the recent production of varieties of optical glass unknown a century ago. Fraunhofer was born in Straubing, Bavaria, March 6, 1787. Self-educated like Herschel, he attained to an extraordinary combination of theoretical and practical knowledge that went far in laying the foundations of astrophysics.

The first mapping of the solar spectrum, the invention of the diffraction grating and its application to determining the wave length of light, the first exact investigation of the refraction and dispersion of glass and other substances, the invention of the objective prism, and its use in studying the spectra of stars and planets, the recognition of the correspondence of the sodium lines to the D lines in the sun, and the earliest suggestion of the diffraction theory of resolution later worked out by Lord Rayleigh and Professor Abbé, make a long list of notable achievements.

To these may be added his perfecting of the achromatic telescope, the equatorial mounting and its clockwork drive, the improvement of the heliometer, the invention of the stage micrometer, several types of ocular micrometers, and the automatic ruling engine.

He died at the height of his creative powers June 7, 1826, and lies buried at Munich under the sublime ascription, by none better earned, Approximavit Sidera.

From Fraunhofer’s time, at the hands of Merz his immediate successor, Cauchoix in France, and Tully in England, the achromatic refractor steadily won its way. Reflecting telescopes, despite the sensational work of Lord Rosse on his 6-foot mirror of 53 feet focus (unequalled in aperture until the 6-foot of the Dominion Observatory seventy years later), and the even more successful instrument of Mr. Lassell (4 feet aperture, 39 feet focus), were passing out of use, for the reason already noted, that repolishing meant refiguring and the user had to be at once astronomer and superlatively skilled optician.

These large specula, too, were extremely prone to serious flexure and could hardly have been used at all except for the equilibrating levers devised by Thomas Grubb about 1834, and used effectively on the Rosse instrument. These are in effect a group of upwardly pressing counterbalanced planes distributing among them the downward component of the mirror’s weight so as to keep the figure true in any position of the tube.

Such was the situation in the 50’s of the last century, when the reflector was quite unexpectedly pushed to the front as a practical instrument by almost simultaneous activity in Germany and France. The starting point in each was Liebig’s simple chemical method of silvering glass, which quickly and easily lays on a thin reflecting film capable of a beautiful polish.

The honor of technical priority in its application to silvering telescope specula worked in glass belongs to Dr. Karl August Steinheil (1801-1870) who produced about the beginning of 1856 an instrument of 4-inch aperture reported to have given with a power of 100 a wonderfully good image. The publication was merely from a news item in the “Allgemeine Zeitung” of Augsburg, March 24, 1856, so it is little wonder that the invention passed for a time unnoticed.

Early the next year, Feb. 16, 1857, working quite independently, exactly the same thing was brought before the French Academy of Sciences by another distinguished physicist, Jean Bernard Léon Foucault, immortal for his proof of the earth’s rotation by the pendulum experiment, his measurement of the velocity of light, and the discovery of the electrical eddy currents that bear his name.

Fig. 24.—Dr. Karl August Steinheil.

Fig. 25.—Jean Bernard Léon Foucault.
The Inventors of the Silver-on-Glass Reflector.

To Foucault, chiefly, the world owes the development of the modern silver-on-glass reflector, for not being a professional optician he had no hesitation in making public his admirable methods of working and testing, the latter now universally employed. It is worth noting that his method of figuring was, physically, exactly what Jesse Ramsden (1735-1800) had pointed out in 1779, (Phil. Tr. 1779, 427) geometrically. One of Foucault’s very early instruments mounted equatorially by Sécrétan is shown in Fig. 26.

Fig. 26.—Early Foucault Reflector.

The immediate result of the admirable work of Steinheil and Foucault was the extensive use of the new reflector, and its rapid development as a convenient and practical instrument, especially in England in the skillful hands of With, Browning, and Calver. Not the least of its advantages was its great superiority over the older type in light-grasp, silver being a better reflector than speculum metal in the ratio of very nearly 7 to 5. From this time on both refractors and reflectors have been fully available to the user of telescopes.

In details of construction both have gained somewhat mechanically. As we have seen, tubes were often of wood, and not uncommonly the mountings also. At the present time metal work of every kind being more readily available, tubes and mountings of telescopes of every size are quite universally of metal, save for the tripod-legs of the portable instruments. The tubes of the smaller refractors, say 3 to 5 inches in aperture, are generally of brass, though in high grade instruments this is rapidly being replaced by aluminum, which saves considerable weight. Tubes above 5 or 6 inches are commonly of steel, painted or lacquered. The beautifully polished brass of the smaller tubes, easily damaged and objectionably shiny, is giving way to a serviceable matt finish in hard lacquer. Mountings, too, are now more often in iron and steel or aluminum than in brass, the first named quite universally in the working parts, for which the aluminum is rather soft.

The typical modern refractor, even of modest size, is a good bit more of a machine than it looks at first glance. In principle it is outlined in Fig. 5, in practice it is much more complex in detail and requires the nicest of workmanship. In fact if one were to take completely apart a well-made small refractor, including its optical and mechanical parts one would reckon up some 30 to 40 separate pieces, not counting screws, all of which must be accurately fitted and assembled if the instrument is to work properly.

Fig. 27.—Longitudinal Section of Modern Refractor.

Fig. 27 shows such an instrument in section from end to end, as one would find it could he lay it open longitudinally.

A is the objective cap covering the objective B in its adjustable cell C, which is squared precisely to the axis of the main tube D. Looking along this one finds the first of the diaphragms, E.

These are commonly 3 to 6 in number spaced about equally down the tube, and are far more important than they look. Their function is not to narrow the beam of light that reaches the ocular, but to trap light which might enter the tube obliquely and be reflected from its sides into the ocular, filling it with stray glare.

No amount of simple blackening will answer the purpose, for even dead black paint such as opticians use reflects at very oblique incidence quite 10 to 20 per cent of the beam. The importance of both diaphragms and thorough blackening has been realized for at least a century and a half, and one can hardly lay too much stress upon the matter.

The diaphragms should be so proportioned that, when looking up the tube from the edge of an aperture of just the size and position of the biggest lens in the largest eyepiece, no part of the edge of the objective is cut off, and no part of the side of the tube is visible beyond the nearest diaphragm.

Going further down the tube past a diaphragm or two one comes to the clamping screws F. These serve to hold the instrument to its mounting. They may be set in separate bases screwed in place on the inside of the tube, or may be set in the two ends of a lengthwise strap thus secured. They are placed at the balance point as nearly as may be, generally nearer the eye end than the objective.

Then, after one or more diaphragms, comes the guide ring G, which steadies the main draw tube H, and the rack I by which it is moved for the focussing in turning the milled head of the pinion J. The end ring K of the main tube furnishes the other bearing of H, and both G and K are commonly recessed for accurately fitted cloth lining rings L, L, to give the draw tube the necessary smoothness of motion.

For the same reason I and J have to be cut and fitted with the utmost exactness so as to work evenly and without backlash. H is fitted at its outer end with a slide ring and tube M, generally again cloth lined to steady the sliding eyepiece tube N. This is terminated by the spring collar O, in which fits the eyepiece P, generally of the two lens form; and finally comes the eyepiece cap Q set at the proper distance from the eye lens and with an aperture of carefully determined size.

One thus gets pretty well down in the alphabet without going much into the smaller details of construction. Both objective mount and ocular are somewhat complex in fact, and the former is almost always made adjustable in instruments of above 3 or 4 inches aperture, as shown in Fig. 28, the form used by Cooke, the famous maker of York, England. Unless the optical axis of the objective is true with the tube bad images result.

Fig. 28.—Adjustable Cell for Objective.

To the upper end of the tube is fitted a flanged counter-cell c, to an outward flange f, tapped for 3 close pairs of adjusting screws as s₁, s₁₁ spaced at 120° apart. The objective cell itself, b, is recessed for the objective which is held in place by an interior or exterior ring d. The two lenses of the achromatic objective are usually very slightly separated by spacers, either tiny bits of tinfoil 120° apart, or a very thin ring with its upper edge cut down save at 3 points.

This precaution is to insure that the lenses are quite uniformly supported instead of touching at uncertain points, and quite usually the pair as a whole rests below on three corresponding spacers. Of each pair of adjusting screws one as 1 in the pair s₁₁ is threaded to push the counter cell out, the adjacent one, 2, to pull it in, so that when adjustment is made the objective is firmly held. Of the lenses that form the objective, the concave flint is commonly at the rear and the convex crown in front.

At the eye end the ocular ordinarily consists of two lenses each burnished into a brass screw ring, a tube, flange, cap, and diaphragm arranged as shown in Fig. 29. There are many varieties of ocular as will presently be shown, but this is a typical form. Figure 30 shows a complete modern refractor of four inches aperture on a portable equatorial stand with slow motion in right ascension and diagonal eye piece.

Reflectors, used in this country less than they deserve, are, when properly mounted, likewise possessed of many parts. The smaller ones, such as are likely to come into the reader’s hands, are almost always in the Newtonian form, with a small oblique mirror to bring the image outside the tube.

Fig. 29.—The Eye-Piece and its Fittings.

The Gregorian form has entirely vanished. Its only special merit was its erect image, which gave it high value as a terrestrial telescope before the days of achromatics, but from its construction it was almost impossible to keep the field from being flooded with stray light, and the achromatic soon displaced it. The Cassegranian construction on the other hand, shorter and with aberrations much reduced, has proved important for obtaining long equivalent focus in a short mount, and is almost universally applied to large reflectors, for which a Newtonian mirror is also generally provided.

Figure 31 shows in section a typical reflector of the Newtonian form. Here A is the main tube, fitted near its outer end with a ring B carrying the small elliptical mirror C, which is set at 45° to the axis of the tube. At the bottom of the tube is the parabolic main mirror D, mounted in its cell E. Just opposite the 45° small mirror is a hole in the tube to which is fitted the eye piece mounting F, carrying the eyepiece G, fitted to a spring collar H, screwed into a draw tube I, sliding in its mounting and brought to focus by the rack-and-pinion J.

Fig. 30.—Portable Equatorial Refractor (Brashear).

At K, K, are two rings fixed to the tube and bearing smoothly against the rings L L rigidly fixed to the bar M carried by the polar axis of the mount. The whole tube can therefore be rotated about its axis so as to bring the eye piece into a convenient position for observation. One or more handles, N, are provided for this purpose.

Fig. 31.—Longitudinal Section of Newtonian Reflector.

Brackets shown in dotted lines at O, O, carry the usual finder, and a hinged door P near the lower end of the tube enables one to remove or replace the close fitting metal cover that protects the main mirror when not in use. Similarly a cover is fitted to the small mirror, easily reached from the upper end of the tube. The proportions here shown are approximately those commonly found in medium sized instruments, say 7 to 10 inches aperture. The focal ratio is somewhere about F/6, the diagonal mirror is inside of focus by about the diameter of the main mirror, and its minor axis is from ⅕ to ¼ that diameter.

Fig. 32.—Reflector with Skeleton Tube (Brashear).

Note that the tube is not provided with diaphragms. It is merely blackened as thoroughly as possible, although stray light is quite as serious here as in a refractor. One could fit diaphragms effectively only in a tube of much larger diameter than the mirror, which would be inconvenient in many ways.

A much better way of dealing with the difficulty is shown in Fig. 32 in which the tube is reduced to a skeleton, a construction common in large instruments. Nothing is blacker than a clear opening into the darkness of night, and in addition there can be no localized air currents, which often injure definition in an ordinary tube.

Fig. 33.

Instruments by different makers vary somewhat in detail. A good type of mirror mounting is that shown in Fig. 33, and used for many years past by Browning, one of the famous English makers. Here the mirror A, the back of which is made accurately plane, is seated in its counter-cell B, of which a wide annulus F, F, is also a good plane, and is lightly held in place by a retaining ring. This counter cell rests in the outer cell C on three equidistant studs regulated by the concentric push-and-pull adjusting screws D, D, E, E. The outer cell may be solid, or a skeleton for lightness and better equalization of temperature.