The cover image was created by the transcriber and is placed in the public domain.

THE WONDERS OF OPTICS.

Spectrum showing the absorptive power of Sodium vapour (Fig. 6).

Solar Spectrum (Fig. 5).

Action of a prism on a ray of light (Fig. 7).
Eng.^d by A. Robin N.Y.

THE

WONDERS OF OPTICS.

BY

F. MARION.

TRANSLATED FROM THE FRENCH, AND EDITED BY

CHARLES W. QUIN, F.C.S.

ILLUSTRATED WITH SEVENTY ENGRAVINGS ON WOOD, AND A COLOURED FRONTISPIECE.

NEW YORK:

CHARLES SCRIBNER’S SONS,

SUCCESSORS TO

SCRIBNER, ARMSTRONG, & CO.

PREFACE.

The present work needs but little introduction to the English public. The author, M. F. Marion, who holds a high official scientific position in Paris, is well known, especially in Europe, as a popular writer on the “Wonders of Optics,” and kindred subjects. As a rule, the original text has been strictly adhered to by the Translator, but in a few instances certain anecdotes of a local character have been altered so as to be more generally applicable, or condensed to make room for the chapter on the Spectroscope, which is entirely original.

CONTENTS.

PART I.

THE PHENOMENA OF VISION.

CHAPTER I.

CHAPTER II.

CHAPTER III.

CHAPTER IV.

CHAPTER V.

CHAPTER VI.

CHAPTER VII.

PART II.

THE LAWS OF LIGHT.

CHAPTER I.

CHAPTER II.

CHAPTER III.

CHAPTER IV.

CHAPTER V.

CHAPTER VI.

CHAPTER VII.

CHAPTER VIII.

CHAPTER IX.

PART III.

NATURAL MAGIC.

CHAPTER I.

CHAPTER II.

CHAPTER III.

CHAPTER IV.

CHAPTER V.

CHAPTER VI.

CHAPTER VII.

CHAPTER VIII.

CHAPTER IX.

CHAPTER X.

LIST OF ILLUSTRATIONS.

THE WONDERS OF OPTICS.

PART I.

THE PHENOMENA OF VISION.

CHAPTER I.
THE EYE.

The Eye is at once the most wonderful and the most useful of all our organs of sense. It is especially by means of the eye that we gain a knowledge of the exterior world. Our other senses are far more limited in their action: thus the sense of touch only extends to objects within our reach; the sense of taste is only a delicate and exquisite modification of the sense of touch; the sense of smell can only be exercised on substances that are close to us; and the use of our ears is limited by the distance at which the loudest sound ceases to impress them. But the eye has the privilege of extending its dominion, whether for mere enjoyment or for serious instruction, far beyond the limits of this little world. Not only is it the origin of all our ideas upon every object that comes within its ken; not only does it reveal to us our own position and that of our surroundings; but, thanks to the discoveries of modern science, it is able to admire, on the one hand, a world of infinite minuteness that remained unknown to us for centuries, and, on the other, the immeasurable immensity of the starry universe.

Admirable as the eye undoubtedly is through the possession of the power of vision, it is also capable of enchanting us by its own particular beauties. Not to speak of its internal mechanism, which we shall consider very fully by and by, let us for a moment examine its outward appearance. Have you never, dear reader, been enchanted with a pair of soft and gentle eyes, or with a couple of black orbs veiled with long dark lashes, or with those wondrous eyes that rival the heavens in colour and depth, shedding on you rays of light whose mute eloquence was irresistible? If it be true that man’s face is the canvas upon which the affections and desires of his mind are depicted as soon as they are formed, the eyes are unquestionably the central point of the picture, and it is in them, as in a looking-glass, that every sentiment that passes across our brain is reflected.

When the mind is undisturbed, says Buffon, all the parts of the face are in a state of repose; their proportion, unity, and general appearance indicate the pleasing harmony of our thoughts and the perfect calmness of our mind; but when we are agitated, the human face becomes a living picture, in which the passions that disturb us are depicted with equal force and delicacy, a picture in which every emotion is expressed by a stroke, every action by a letter, so to speak; in which the quickness of the impression outstrips the will, and reveals by the most sympathetic signs the image of our secret trouble.

It is more especially in the eyes, adds this great naturalist, that these signs are manifested and recognised. The eye is connected with the mind more than any other organ: it seems almost to be in contact with it and to participate in all its movements; it expresses in obedience to it the strongest passions and the most tumultuous emotions, as well as the gentlest thoughts and most delicate sentiments, and reproduces them in all their force and purity just as they have sprung into existence; it transmits them with exquisite rapidity even to the minds of others, where they once more become impressed with all their original fire, movement, and reality. The eye both receives and reflects the light of thought and the warmth of sentiment, and is at once the sense of the mind and the tongue of the intellect. Persons who are short-sighted, or who squint, have much less of this external intelligence that dwells in the eye. It is only the stronger passions that can bring the other features of the face into play, that are depicted on their physiognomy; and the effects of fine thought and delicate feeling are rendered apparent with much greater difficulty.

The elegant author of L’Histoire Naturelle rightly thinks that we are so accustomed only to see things from the outside, that we are hardly aware how much this exterior view of everything influences the judgment of even the gravest and most thoughtful of us. Thus we are apt to set down a man as unintellectual whose physiognomy does not particularly strike us; and we allow his clothes, and even the manner in which he wears his hair, to influence our judgment of him. Hence, our author goes on to say, not wholly without some show of reason, that a man of sense ought to look upon his clothes as part of himself, because they really are so in the eyes of others, and play an important part in the general idea that is formed of him who wears them.

The vivacity or languor of the movement of the eyes forms one of the chief characteristics of facial expression, and their colour helps to render this characteristic more striking. The different colours seen in the eye are dark hazel, or black, as it is generally called, light hazel, blue, greenish grey, dark grey, and light grey. The velvety substance which gives the colour to the iris is arranged in little ramifications and specks, the former being directed towards the centre of the eye, the latter filling up the gaps between the threads. Sometimes they are both arranged in so regular a manner that instances have been known in which the irises of different eyes have appeared to be so much alike that they seemed to have been copied from the same design. These little threads and specks are held together by a very fine network.

The commonest colours seen in the eye are hazel and blue, and it mostly happens that both these colours are found in the same individual, giving rise to that peculiar greenish-grey hue that is far from being uncommon. Buffon thinks that blue and black eyes are the most beautiful, but this of course is a matter of taste. It is true that the vivacity and fire which play so important a part in giving character to the eye, are more perceptible in dark eyes than in those whose tints are lighter; black eyes, therefore, have greater force of expression, while in blue eyes there is more softness and delicacy. In the former we see a brilliant fire, which sparkles uniformly on account of the iris, which is of the same colour throughout, giving in all parts the same reflection; but a great difference may be perceived in the intensity of the light reflected from blue eyes, from the fact of the various tints of colour producing different reflections. There are some eyes that are remarkable for being almost destitute of colour, and appear to be constituted in an abnormal manner. The iris is tinted with shades of blue and grey of so light a hue that it appears quite white in some places. The shades of hazel in such eyes are so light that they are hardly distinguishable from grey and white, in spite even of the contrast of colour.

For our part, we think that the beauty of the eye consists not so much in its colour, or even in its harmony with the rest of the face, but in its expression.

There are also numerous instances of green eyes. This colour is, of course, much less frequent than blue, grey, or hazel. It often happens, too, that the two eyes vary in colour in the same individual. This defect is not confined to the human species, being shared by the horse and the cat. In most other animals the colour of the two eyes is always similar. The colour of the eye in most animals is either hazel or grey. Aristotle imagined that grey eyes were stronger than blue, that those persons whose eyes are prominent cannot see so far as others, and that brown eyes are less valuable in the dark than those of another tint; but modern investigations have failed to bear out the ancient philosopher’s ideas with regard to the human eye.

Although the eye appears to move about in every direction, it has in reality only one movement, that of rotation round its centre, by means of which the eyeball rises or falls, or passes from side to side at will. In man the eyes are parallel with each other in relation to their axes; he can consequently direct them at pleasure upon the same object: but in most animals this parallelism is wanting. In some cases the eyes of animals are set almost back to back, rendering it impossible for them to see the same object with both eyes at once.

Buffon makes the remark, that after the eyes, the eyebrows contribute more strongly than any other part of the face towards giving character to the physiognomy, being, inasmuch as they differ in their nature from the other features, more apparent by contrast, and hence strike us more than any other portion of the countenance. They are, in fact, a shadow in the picture, bringing its colour and drawing into strong relief. The eyelashes also contribute their effect; when they are long and thick, they overshadow the eye, making its glance appear softer and more beautiful. The ape is the only other animal besides man that possesses two eyelashes, the rest having them only on the upper eyelid. Even in man they are more abundant in the upper eyelid than in the lower. The eyebrows have but two movements, upward and downward, governed by the muscles of the forehead. In the action of frowning we not only lower them, but move them slightly towards each other. The eyelids serve to protect the eyeball, and keep the cornea from becoming dry. The upper eyelid has the power of raising and lowering itself, the lower one being almost destitute of movement. Although the motion of the eyelids is an effort of will, there are times when it is impossible to keep them open, as for instance when we are overpowered by sleep, or when the eyes are suddenly subjected to the effects of strong light. The eyelid is a most admirable arrangement for the protection of the eye, and it is almost impossible to admire this provision of nature too much, even when we confine ourselves to an outward examination of it. It is not merely the outward mechanism and motion of the eyelids, nor the colour of the eyes, that constitutes their beauty; we have already said that the leading characteristic of the eye was expression. It is this expression which causes the eye to appear to speak, to fire up suddenly, to sparkle with flashes of light, to languish or conceal itself underneath its lashes, to raise itself with inspiration, or to pierce the abyss of thought, just according to the particular sentiment governing the mind at the moment. Hence it is expression that constitutes the true beauty of the eye: every one knows instances of eyes which, while at rest, would never be noticed by anybody, but which, when once animated by intense eloquence, lend to the voice of their possessor an unexpected power, which moves and transports the listener to an extent infinitely beyond that resulting from the simple spoken words.

Enough, however, has been said upon the external aspect of the human eye; we will, therefore, at once endeavour to penetrate the circle in which are contained the wonders that this little book is intended to describe. The object of these lines is not so much to describe the beauty of man’s glances, nor the value of his senses, but rather to make known those illusions to which the most sagacious of all his senses is apt to fall a prey. But before entering the temple it was but right to have bestowed a little admiration upon the façade. By the way, as we are about to describe many illusory wonders, do not let us commence by deceiving ourselves with regard to our first marvel—the eye itself. A great philosopher calls the eyes the windows of the soul, and, although meant as a poetical image, the saying is not far from the truth; for the optic nerve by which we see external objects, is an extension of the nerves of the brain, whose functions and actions are an unfathomable mystery.

CHAPTER II.
THE STRUCTURE OF THE EYE.

Of all the senses, says an ardent admirer of nature, the sight is certainly that which furnishes the mind with the quickest and most widely-extended perceptions. It is the source of the richest treasures of the imagination, and of our ideas of the beauty, order, and unity of the world around us. How unhappy are those whom a hard fate has deprived of the sense of sight from their birth! Alas! the finest day and the darkest night differ in nothing as far as they are concerned; the light of heaven never brings joy into their hearts. The enamelled beauties of a bed of flowers, the varied plumage of the peacock, the glories of the rainbow are alike unknown to them. They cannot contemplate from the mountain height the beauties of the valley beneath; the fields golden with the harvest, the meadows smiling with verdure, and watered by winding rivers, and the habitations of man dotted about here and there over the surface of this magnificent picture. To them is unknown the sight of the mighty ocean; and the innumerable legions of the cloud army of Heaven are to them as if they did not exist. The impenetrable obscurity which surrounds them allows them neither the contemplation of what is grandest in man’s outward aspect, nor even the admiration of those qualities which they themselves would hold most dear.

A strong sentiment of pity should, therefore, animate the breast of every right-thinking man, when he considers the unhappy condition of those who are born blind.

The eye infinitely surpasses in its complexity and beauty of structure all the other organs of sense, and is most unquestionably the most marvellous object that the human mind is capable of examining and understanding. Let us first examine the external parts of this wonderful organ. With what a singular system of entrenchments and defences do we find the eye provided! It is itself placed in the head at a certain depth, and surrounded on all sides by solid bone, so that it is only with the greatest difficulty that it is hurt by accident from without. The eyebrows also play their part as protection to the eye, and prevent the perspiration from entering and irritating the organ. The eyelids too are always ready to rush to the rescue, whether to protect the eye from outward attacks, or to shade it from too strong a light during sleep. The eyelashes not only add to the beauty of the eye, but they shade it from the too brilliant light of the sun, and act as advanced guards to prevent the entrance of dust or any other foreign body with which the eyes might be injured.

But its internal structure is still more admirable. The globe of the eye is almost spherical and measures nearly one inch in diameter. [Fig. 1] is a view of the eyeball, showing the details of its structure; the various membranes surrounding it have been cut away in order that it may be better examined. If we commence our examination by the exterior portion of the front, we shall first find immediately beneath the eyelashes a perfectly transparent membrane (C), called the cornea. It is a prolongation of the hard opaque external coating of the eye, called the sclerotic membrane, and marked S in the figure. The cornea is sufficiently hard in its nature to present a strong resistance to any violence from without.

Immediately beneath the cornea and in contact with it is the aqueous humour, a thin transparent liquid occupying a small portion of the front of the eye.

Next comes the iris, a circular disc perforated with a round hole in the middle, and coloured with various shades of blue, brown, and grey.

Fig. 1.—Section of the Eye.

The opening in the centre, which appears like a black spot when the eye is examined, is not really an object, but simply an aperture, capable of changing its size according to the quantity of light striking the eye. This change of size in the opening, or pupil, as it is popularly called, is effected by the contraction or expansion of the iris, which thus possesses the peculiar property of exactly proportioning the amount of light that enters the eye, so that there is never too much or too little. It is through the pupil that the rays of light proceeding from the various objects around us pass into the interior of the eye, and form an image upon the retina, as will be afterwards explained.

Immediately behind the pupil is O, a bi-convex lens to transmit the rays of light to the retina. It is generally called the crystalline lens.

From the crystalline lens to the back of the eyeball, is a space more or less globular in form, containing a gelatinous diaphanous mass somewhat resembling white of egg in appearance, and called the vitreous humour.

Behind the vitreous humour, and immediately opposite the pupil and lens, is the most delicate and important of all the membranes of the eye, the retina, which serves as a screen whereon are received the images of the objects around us. This membrane is an expansion of the optic nerve N leading from the brain, and lines the whole of the interior of the eye. The eye is also enveloped in a second membrane (C), called the choroid, which is impregnated with a black pigment. Round this is wrapped a third membrane, the sclerotic (S), which unites with the cornea in front of the eyeball.

The crystalline lens through which all the rays pass before they reach the retina, possesses the marvellous power of being able to modify its curvature in such a manner as to adapt itself to the distance of the object seen, and thus throw a distinct image on the retina. When we come to talk of the properties of lenses, we shall see that the focus of a lens differs for objects at different distances; if, therefore, the eye were not provided with some such means for altering the focus of the crystalline lens, we should only see objects distinctly at one particular point. The crystalline lens consists of infinite numbers of extremely thin transparent little plates, each of which is in itself composed of fine fibres so united together as to be capable of a small degree of compression or extension. Hence the power of the lens to alter its form according to circumstances. It is calculated that the human eye contains over five millions of the laminæ above referred to. With such wonders is the world of nature replete,—wonders that we daily and hourly pass by without examination.

It is by means of this ingenious and inimitable structure of the eye that external objects pass from the domain of the material world into that of the mind, and become accessible to every faculty of our brain. Of its own accord, and without apparently any effort of our own will, does this marvellous mechanism adapt itself to all the variations of distance and intensity of light, a power possessed by no instrument as yet constructed by the hand of man—being capable, as it is, of distinguishing instantaneously between the distance of the remotest nebulæ and that of the letters forming this page. This wonderful organ, writes Brewster, may be considered as being the sentinel that guards the passage between the world of matter and that of mind, and as the medium through which they interchange all their communications. The optic nerve perceives the objects written on the retina by the hand of nature, and conveys them to the brain in all their integrity of form and colour.

The path of the rays of light and the formation of images upon the retina are shown in the preceding figure. At first sight it will be perceived that the objects thereon depicted are in a reversed position, that is to say, when we look at a view similar to that shown in [fig. 2], we should find, if we had any means of observing the positions of objects reflected on our retina, that the flock of sheep coming up the road were at the top of the eye, while the trees, the roof of the house, and the chimney were in the contrary position. Similar reversed images may be seen in dark rooms, by holding a screen before any little crack or pinhole in the door or shutter of the room. In [fig. 2] the keyhole of the door is represented as playing the part of a lens. The author, in common with almost every other boy, observed this fact at a very early age, and the idea immediately struck him that it would be only necessary to fix these images to procure exact representations of natural scenery; but in making inquiries into the subject, he found that his juvenile observations had been made a little too late, photography having already gained the end he intended striving for.

Fig. 2.—A Camera Obscura.

Seeing that the images of all objects appear on our retina upside down, the student is naturally disposed to ask how it happens that we do not see them in that position. Physiologists and natural philosophers have advanced numerous theories on the subject. Some, with Buffon, admit at once that it is by habit and education of the eye that we see objects unreversed. Others, like the great physiologist Müller, imagine that as we see everything upside down, and not a single object only, we have no points of comparison, and practically ignore the reversal. The truth, however, appears to be that it is the brain, and not the eye, that possesses the power of determining the real position of what we see. That the eye alone has no power of determining the positions of objects by itself, may be easily proved by showing a person an astronomical object, such as the moon through a telescope. Unless the observer has been already familiarized with the appearance of our satellite, he will not know whether the image he sees is reversed or not. It is the brain, therefore, and the brain only, that has the power of determining the position of objects around us, without taking into consideration the reversed picture of them that is depicted on our retina. The student who takes an interest in the structure of this important organ, would do well to procure a sheep’s or bullock’s eye from the butchers, and dissect it carefully with a sharp penknife and pair of scissors. The image formed on the retina may be easily seen by cutting away the sclerotic and choroid coatings at the back of the eye.

The ordinary distance of distinct vision for small objects, such as the letters of a book, is from ten to twelve inches. But possibly there do not exist two pairs of eyes in the world whose foci are the same. Even in the same individual it frequently happens that the focal length of the eyes differs considerably. In some persons the focus of the eye is so reduced that they are obliged to bring the object they are examining within six, and even four inches of their eyes, before they can see it. This defect is known ordinarily as short sight, and results from the too great convexity of the cornea and crystalline lens. It is corrected by wearing spectacles with concave glasses. Others again, on the contrary, place the book or object they are looking at, at a greater distance from the eye than that named. Such people are called long-sighted, and the defect results from the too great flatness of the cornea and the crystalline lens. The fault is of course corrected by the use of spectacles containing convex lenses.

Long-sightedness is generally the result of old age, and it may be taken as a fact that the older we grow the flatter becomes the crystalline lens. Hence short-sighted people have been known to recover their sight perfectly as they advance in years through the natural process of the flattening of the crystalline lens. These matters, however, will be more fully treated of when we begin to speak of the properties of lenses of different forms and curvatures.

CHAPTER III.
THE ERRORS OF THE EYE.

It is with our own organization that we shall commence our task of exposing the illusions that we shall meet with during our optical experiments,—in fact with that wonderful and important organ of our body that we are apt to look upon as sure and infallible, but which we shall find is deceiving us constantly, and hourly proving the fallacy of the popular saying, that “every one must believe his own eyes.” In ancient times there existed a school of sceptics who doubted everything beginning with Pyrrho, the great theorist, and ending with the follower of his school who doubted the existence of muscular force even after he had received a sound box on the ear from an opponent of his system of philosophy. If any of our readers were to become followers of Pyrrho, they might easily do so when considering the numberless illusions we shall describe to them, if they did not remember that if our senses are subject to error, we have a brain to set them right: our mind, if logical and well regulated, soon discovers errors of observation, and speedily places our judgment on the most solid basis. We shall find endless instances of this throughout our little book. If we are dazzled with illusions from time to time we shall as often recover ourselves; and no matter how beautiful or interesting these deceptions may appear, we shall speedily be able to convince ourselves that they are unreal. In this chapter we shall only speak of those errors of the eye of which we have actually lost all cognizance, so effectually has our judgment succeeded in counteracting their influence.

We all know that the first thing a child does with its eyes, even when it is only five or six weeks old, is to turn them towards the most brilliant object within its reach. Instinctively and without being aware of it, the child’s eye seems to seek the light. The whole of nature, from the lowest plant to the baby in the cradle, appears more or less endowed with this instinct of turning towards the light.

From the time that children begin to distinguish objects, their eyes are liable to be affected by two causes of error. Before being able to judge of the position of things surrounding them, they see everything upside down; they consequently acquire a false impression of the position of objects. The next cause of error that is likely to mislead them is the fact of their seeing everything double, a separate image of everything being formed on each eye; and it can only be by the experience gained through the sense of touch that they can acquire the knowledge necessary to rectify these errors, and see those objects single which appear to them double. This error of sight, as well as the first one, is set right so easily in the end, that although in reality we see everything double and upside down, we imagine that we see them single, and in their proper positions, a state of things brought about entirely through another sense exercising its power over our judgment; and it is hardly too much to say that, if that sense were deprived of the power of feeling, our eyes would deceive us, not only as to the number, but the position of the objects within our view.

It is very easy to convince ourselves that we really see objects double, although we imagine them to be only single. We have only to look at the same object first with the right eye, and we shall see it directly against some portion of the wall of the room in which we are sitting; then looking at it with the left eye, we shall see that it covers a different part of the wall. This experiment is easily tried, and is very convincing. Thus we see that an image is formed on both eyes, and we consequently see the object, whatever it may be, repeated twice. By degrees, however, the eyes gain the power of converging their axes on objects at different distances, so that they fall on similar portions of each retina, and so convey a single impression to the brain. Thus, for instance, if we look at a pencil held up at arm’s length, and then, without changing the position of the eyeball, look at some distant object, we shall see it double. Let us, however, converge the eyes upon it, and the two images unite. Reverse the experiment by now looking at the pencil without converging the eyes upon it, and we shall see that object double in its turn. The same thing happens if we push aside one of the eyes with the finger while looking at any object. During severe illness it often happens that the patient from extreme weakness loses the power of convergence, and consequently sees every thing double, and we continually see children’s faces wearing a most distressing appearance through having temporarily lost the power of moving the muscles of the eye. It is a common expression to use in speaking of drunken people, that they see double, but the saying, unlike many others, is no metaphor; when a man gets drunk he loses his power over the muscles of his eye, just as he does over those that sustain his body, and the instinctive closing of one eyelid, in order that he may see objects single, is an effort of his weakened judgment to set things right once more.

While on this subject we may mention the experiment made by the famous English surgeon Cheselden upon a boy who was born blind, and upon whom he operated successfully.

This boy, who was thirteen years old at the time that Cheselden restored to him the sense of sight, was not born absolutely blind, his affliction having been caused by a cataract or film spread over the eyeball, which allowed him to distinguish night from day, or black from white or scarlet when placed in a very good light, although he was unable to perceive the form of things around him. At first Cheselden operated on a single eye, perfectly restoring its power; but so little idea of distance did the new sense convey to the boy’s mind that for a long time he imagined that everything touched his eyeball, just as those he felt touched his skin, and it was only by the sense of touch that he could persuade himself of the fallacy of his supposition. At first he had no perception of form whatever, and could only recognize objects he had already been familiar with after he had felt them all over. He was a long time, for instance, before he could distinguish between the dog and the cat without touching them, and was greatly surprised to find that the persons and things he had liked best when blind were not always the pleasantest to his newly acquired sense. His ideas of size, too, were all at fault, and he could not, for a long time, be made to understand how his father’s picture could be got into the back of his mother’s watch; even after he had possessed his sight for a comparatively long time, he could still only recognise people he had known during his blindness by touching their faces. Whenever he saw a new object he looked at it attentively for some time, in order, as it were, to learn its form by heart; but his memory was at first so overtaxed that he continually forgot his visual impressions, and mistook one thing for another. He was more than two months before he could appreciate form as depicted in a painting or drawing, having hitherto learned to consider pictures as flat objects. When, however, he began to understand the power of light and shade in producing the representations of solid objects, he was often extremely surprised to find the surface on which they were depicted quite flat when he touched it. The same thing frequently happens to ourselves, when looking at the photographs of bas-reliefs for instance. If these objects be well photographed, with the proper arrangement of light and shade, the illusion is so complete that the finger involuntarily touches the paper to feel if the surface is not really raised. In the Bourse at Paris there are some figures painted to represent bas-reliefs in so wonderful a manner, that numberless bets have been made, lost and won, over them. When feeling such representations of solid objects, the boy would often ask those around him which of his senses was deceiving him, his sight or his touch.

At first he saw everything of an enormous size, but as he saw things larger than those around him, he found the latter diminish. He also imagined that there was nothing beyond the room he was in, and could not be brought to comprehend how the house could be larger. When the sight of the second eye was restored to him a year afterwards, he at first saw every object of an enormous size, just as in the case of the first eye; but as he had now the perfectly educated organ to help him as well as his sense of touch, he soon began to see things under their natural appearances.

While he was in ignorance of what sight really meant, he was not particularly anxious to undergo the operation, saying that he did not think it possible to derive more pleasure from things that he liked than he did while he was blind. But now that his sight was restored he found every fresh object a new pleasure. When first he was shown the landscape from the top of a high hill, he was so delighted that he exclaimed that he had found another sense. When his second eye was operated upon, he saw things apparently twice as large with both eyes as with the one already restored to him. Even at first he seemed to have no difficulty in converging the eyes on any object.

These extracts from the history of Cheselden’s patient show us how utterly incapable the eye must be of rightly understanding the number, position, size, and form of objects without frequently correcting our impressions by the aid of the sense of touch.

CHAPTER IV.
OPTICAL ILLUSIONS.

Besides the errors of sight already spoken of, there are other illusions, which are either common to all persons or confined to certain individuals, the knowledge of which will serve as a fitting prelude to the descriptions of those which are artificial.

The following defect, for instance, is one which is little known, but notwithstanding our ignorance of its existence it is nevertheless true that we all suffer from it. There is in every one’s eye a blind spot, totally incapable of experiencing the effects of the rays of light when they impinge upon it. For objects situated opposite to this particular spot we are as completely blind as if we had no eyes at all. To convince yourself of the truth of this assertion it is only necessary to try the following simple experiment.

Place upon a piece of white paper two small wafers, or two blots of ink about an inch and a half apart. Take the sheet in your right hand, and hold it up parallel to the lines of the eyes; shut the left eye, and fix the right eye on the centre of the left wafer or ink-spot. Move the sheet of paper steadily towards the eye, until it is about two inches and a half or three inches’ distance from it, and you will find that in a certain position the other wafer or ink-spot will disappear, although it is evidently still in the field of view. Having discovered this point which differs for different eyes, you will find that if you diminish or increase the distance of the paper you will once more see the missing object. The same thing happens if you move the eye from the centre of the wafer. The same experiment may be repeated with the left eye with a precisely similar result.

It has been found by experiment that this particular blind space exists exactly over the base of the optic nerve, at the spot where it joins the eye. ([Fig. 1]). Thus we see that the nerve which actually conveys the impression of sight to the brain is in itself incapable of being excited by light. In such cases as these Nature seems to laugh at us, and escapes from our grasp just as we are most confident in our power of wresting her secrets from her; indeed we may compare her to a wise and good-natured mother, who, though always amiable and willing to instruct those about her, sometimes smiles when her children fancy they are as learned as she is.

If we do not perceive the constant recurrence of the phenomenon just mentioned, it is because when both eyes are open the object whose image falls on the blind spot in one eye is seen by the other, the insensible portions of each eye being on opposite sides. Not only this: the spot being always situated on the outer and indistinct portion of the image reflected on the retina, we do not take notice of it; for as every one has no doubt observed, it is only the small portion of the object we are looking at exactly opposite the centre of the eye that is perfectly distinct and clear, all the rest being confused in its details, although quite visible.

Again, we may account for our not noticing it by the fact of our seeing clearly only those things which specially attract our attention—a fact first noticed by Mariotte. We see only what we wish to see with our physical eyes, as well as those of our mind. If our attention is attracted by a particular portion of a landscape, we see only that, and nothing else. If it is fixed on some subject that we are contemplating inwardly, we see nothing at all, although our eyes may not only be wide open, but absolutely fixed on some particular object. For instance, suppose a sportsman is out in the fields preceded by his dogs, Bran and Ponto. If he follows the movements of Bran with attention, he becomes the only object animate or inanimate, that depicts itself on his retina. Ponto may jump and caper in vain: he is lost to his master’s eye as much as if he were not there at all; his mind is entirely fixed on the beauty of Bran’s coat, on the fit of his collar, or fifty other things, and he sees nothing else. But let the sportsman begin to think of the number of birds he shot yesterday, or how he will find time to get up to the grouse in Scotland, or of that fine stag he missed when he was last amongst the heather, and dogs, cover, and landscape will fade from his sight as effectually as if he had been struck with blindness. Let him, however, strike his foot against a stump, or let the dogs suddenly begin to point, and he instantly receives back his sight, which but a few moments before he had lost to all intents and purposes.

The phenomena of ocular spectra and complementary colours experienced by every one forms a curious chapter in the history of those illusions which take their origin in the eye itself. Every one has noticed that after looking fixedly at a bright light or a striking colour for a few moments, the eye preserves an impression of the object for a certain time. A very light window looked at intently for several seconds will leave the impression of its cross-bars on the retina for several minutes, the colour of the image changing at every movement of the eye. The same effect may be observed when looking at the setting sun, or a flaring gas light. If the light at which we look is coloured, we shall see the complementary colour in the impression left on the retina. Sir David Brewster was one of the first to notice and experiment upon these very interesting facts.

If we cut out any simple figure, a small cross for instance, in scarlet paper, place it upon a white background and look at it fixedly for a minute or two, we shall find that its tint will gradually become duller. If we now suddenly look at a piece of white paper, we shall see the cross depicted upon it in green, which is the complementary colour to red. It should be explained, that the complementary of any colour is that which is necessary to make white light. Thus, blue, yellow, and red (as we shall find out when we come to speak of the prismatic spectrum), mixed in certain proportions, form white light; consequently the complementary of orange, which is composed of red and yellow, will be blue; of green, which is yellow and blue, red; of purple, which is blue and red, yellow, and vice versâ. The complementary of black is white, and of white, black as a rule; but if the white object be very brilliant, the black spectrum will speedily become coloured. The impression left by the setting sun is of this character. At first, while the eye is open, the image is black, then brownish red, with a light blue border; but if the eye be shut suddenly, it becomes green, with a red border, the brilliancy of colour being apparently in proportion to the strength of the impression. These spectra may be perceived for a long time, if the eye is gently rubbed with the finger now and then. Some eyes are more impressionable in this respect than others, and Beyle gives an instance of an individual who saw the spectrum of the sun for years, whenever he looked at a bright object. A modern instance of this occurred lately to an amateur astronomer who was looking at an eclipse of the sun. He unfortunately used a glass that was not sufficiently smoked, and the image of the sun’s disc, with the black space caused by the intervening moon, remained on his retina for months after. This gentleman’s case afforded an instance of the necessity of attention in order to see any object, for after the first few days he only became sensible of his unfortunate mishap when his attention was called to it by some accidental circumstance. These facts were so inexplicable to Locke, that he consulted Newton on the subject, and was surprised to learn that the great philosopher himself had suffered for several months from a sun-spectrum in the eye.

Without affirming that optical illusions are the cause of all the supposed supernatural appearances of which we have heard so much, there is no doubt that in many instances the eye plays an important part in deluding the brain. The following example, also cited by Beyle, will show this clearly. A horseman dressed in black, and riding a white horse, was trotting along a portion of the road, which through a sudden break in the clouds was brilliantly illuminated by the rays of the sun. The black figure of the man was projected against a white cloud, and the horse appeared doubly brilliant from being seen against the dark-coloured road. A person who was greatly interested in the arrival of the horseman was watching them with great attention, when suddenly the horse and his rider disappeared behind a wood. An instant after the observer was terrified at seeing a white cavalier on a black horse projected on a white cloud at which he was accidentally looking. It may be readily imagined that such an occurrence, followed up by a succession of unusual events,—such as illness, death, or any other series of misfortunes,—might even in the present day add a chapter to the history of the marvellous.

To the illusions to which, like the preceding, we are all subject, may be added those resulting from some abnormal conformation, or some disease of the eye, in those who labour under them. An example of this occurs in the case of double or triple vision, many remarkable instances of which are mentioned by Müller, the celebrated physiologist.

Although, as before explained, the image of an object is depicted at the same time on both our eyes, still we only see one impression, in consequence of the two images being carried to the brain from corresponding portions of the retina. If this relation be disturbed by any cause, or if the eyes are not converged exactly upon the same point, a double image is the result. The first of these facts may be proved by looking at the moon, for instance, with the left eye shut; on suddenly opening it, two images will be seen for an instant. The second is instantly proved by pushing either of the eyes aside with the finger, when looking at any object.

It is necessary, however, to distinguish between these effects and true double vision, as well as a certain defect which exists in the eyes of many people, consisting in the apparent multiplication of distant objects by the same eye. In these cases, there is a superposition of images upon the retina, each having its proper bounds. With the majority of individuals afflicted in this way, it only happens when they look at a very distant object, the moon or stars for instance. There are many, however, who suffer from it in the case of everything they look at, whether far or near. Stephenson, who was affected with it, made it the subject of many interesting experiments. When he looked at a clear mark on a white ground, and gradually walked away from it, not only did the image become indistinct, but it seemed to unfold itself into several, independently of many others much more indistinct, more especially two situated on each side, whose distance increased the farther he walked away. As these latter images became more and more separated, they also became more confused. The image seen by the right eye was a little higher than that seen by the left. Griffin states, that after having used the telescope for any length of time, the eye that he kept shut always saw objects triple and double for some hours afterwards. These phenomena are possibly connected in some way with the disposition of the plates and fibres of which the crystalline lens of the eye is composed.

Semi-vision, or hemiopia as it is called, is much more rare and more difficult to explain than the phenomena of double vision; and consists in the power of being able to see only the right or left half of the object looked at, the separation being vertical when the eyes of the observer are in the same horizontal line. Thus, in looking at the word Newton, the person so afflicted would only see either the letters New or TON according to which half of the eyes were defective.

Wollaston was afflicted with hemiopia on two different occasions; the first time after violent exercise, during two or three hours, when he could see distinctly only the left-hand halves of the objects he was looking at. Both eyes were similarly affected, and the phenomenon only lasted about a quarter of an hour. Twenty years afterwards he suffered again from the same accident, but on this occasion in the contrary manner; that is to say, he only saw the right halves of the objects he was looking at—to use his own words, he could only see the right half of every friend he met. At certain distances from the eye, one of two persons would become invisible, and by simply changing his own position or that of the persons he was near, he could make one or other of them, or indeed both, disappear at will. It must be acknowledged that similar tricks of Dame Nature, due to an unconscious insensibility of the eye, are most singular, and at first sight appear to have a supernatural origin.

Bartholin mentions the case of a hysterical woman who was afflicted with hemiopia horizontally, and saw all natural objects cut in two, the lower halves being invisible. In this instance it was only the left eye that was defective.

Another interesting example of optical illusion is the luminous sensation produced internally when the eye, or the neighbouring parts, are struck or stimulated by friction or electricity. These appearances are experienced even by those who have lost their sight. Müller states that a case was submitted to a legal tribunal to decide whether the luminous sensations which are perceptible when we rub our eyes are really light. The matter in dispute was whether a man who was attacked by robbers in the dark, could see and recognise them by means of the light produced in his eyes by a violent blow on the head; but he does not tell us how the question was decided. With regard to internal causes, Humboldt tells us that a man whose eye had been extirpated, was sensible of luminous appearances whenever he was galvanized. Lincke states that a man whose eye had been removed by a surgical operation, saw next day all kinds of luminous phenomena, which tormented him cruelly with the idea that after all his eye had been saved. When he shut the perfect eye, he fancied he saw with the missing eye circles of fire, persons dancing, and similar appearances for several days. These facts are analogous to those told of persons who have had their legs and arms amputated, but who, notwithstanding, apparently feel pain in their lost limbs.

CHAPTER V.
THE APPRECIATION OF COLOUR.

Most people understand each other sufficiently to agree in their ideas about various colours. Thus every one agrees in saying that poppies are red, that the sky is blue, and the leaves green; but if any one were to assert that the sky was red, that the leaves were blue, and poppies green, who could possibly contradict him?

This statement may appear a paradox, and an absurdity to many of our readers, but it is really a problem that has engaged the attention of many of our greatest philosophers. Who can prove that what I see as yellow may not appear blue to you, or that what you see red is not green to me? You would possibly explain the doubt by saying that because we both agree in calling a buttercup yellow, that we see the same colour. I call a buttercup yellow, because I have learnt since my childhood to give this name to the particular sensation I experience when I look at one of these flowers; but that is no proof that the sensation I feel is similar to that felt by everybody else, and it is not merely possible, but probable, that our personal sensations of colour are essentially different, although the arbitrary words we use to designate them are the same.

It may be remarked in parenthesis, that colour is not an entity, but is simply the effect of certain properties of surface or interior structure possessed by every substance with which we are acquainted. The old saying, that “all cats are black in the dark,” is really a profound philosophical truth, which is not only true of cats but of the reddest rose that ever grew in a garden, the bluest violet that ever was plucked, the prettiest girl that ever was kissed under the mistletoe. It is a sad thing to think of, that when we put the candle out, and step into bed, we become blacker than the blackest negro that was ever emancipated. But without light there can be no colour, for there is no material, so to speak, from which to manufacture it. White light, as we have said before, is made up of red, blue, and yellow, and it is by the absorption of one or all of these that all tints are formed. The surface of a poppy leaf has the power of absorbing all the blue and a little of the yellow, reflecting the whole of the red and the remainder of the yellow, the mixture of the two forming scarlet. The surface of a marigold acts differently; all the blue is absorbed, as in the case of the poppy, and a good deal of the red with it, leaving just a little to brighten up the yellow which is reflected with it. Some substances, white marble for instance, have no power of splitting the light into colours, absorbing some and reflecting others, but reflect the whole of it in its integrity. Others again, like black velvet, absorb nearly the whole, just reflecting sufficient to enable us to see its surface.

We began this chapter by speculating on the probability of our seeing different colours to our neighbours, and we shall now proceed to show that our speculations in that direction are not so absurd as they appear to be at first sight.

The phenomenon of colour-blindness, or the insensibility of the eye to certain colours, has been for many years past a puzzle both to the physiologist and the philosopher. Perhaps the most remarkable case of the sort is that mentioned first by Huddart, and quoted by Sir David Brewster, of a shoemaker named Harris, living at Maryport, in Cumberland, who was utterly incapable of distinguishing any colour at all, and saw everything white, grey or black. The first time that Harris noticed this defect, was when he was about four years old; having found the stocking of a playmate in the street, he returned it to him at his cottage, and noticed that every one said it was a red stocking, but he could not understand why they should call this particular stocking red, as it seemed to him to be like every other. This circumstance remained in his mind, and a few more similar observations confirmed his suspicions that he had some defect of sight that prevented him from seeing as others did. He also observed that other children pretended to distinguish cherries from their leaves by what they called their colour, whilst he could see no difference between them, except those of shape and size. He also noticed that by means of the difference of colour, others could distinguish cherries on a tree at a much greater distance than he could; whilst he, on the contrary, could see other things at greater distances than his companions. Harris had two brothers, whose eyes were similarly defective; one of these, that Huddart examined, mistook green for yellow constantly, and orange for light green.

In the Philosophical Transactions Scott describes a similar defect in his own powers of vision. He states that he was unable to distinguish green, and that the colours known as crimson and pale blue presented no difference of hue. He further confesses his inability to see any difference between bright green and bright red, although he could distinguish between red and yellow, dark blue, and almost every shade of blue, except sky-blue. He goes on to relate how he married his daughter to a worthy young man of his acquaintance, and that the day before the wedding the bridegroom came to his house in a full suit of black, as he thought. He was greatly displeased to see him appear in mourning on such an occasion, and took an opportunity to remonstrate with him on the subject. But what was his surprise to hear his daughter exclaim, in loud tones of counter remonstrance, that she had rarely seen her lover in a coat of such a pretty colour, and that her father’s eyes must deceive him on this as on many other occasions. Scott’s father, his maternal uncle, one of his sisters, and two of his sons had the same defect of sight. Dr. Mitchell mentions the case of a naval officer who for his ordinary uniform chose a blue coat and waistcoat and red trousers, fully believing that they were all of the same colour. A tailor of Plymouth, also mentioned by Dr. Mitchell, mended a black silk waistcoat with a piece of crimson, and another put a red cloth collar to a blue coat. Several celebrated men have suffered from colour-blindness. Amongst them may be mentioned Dugald Stewart, the great philosophical writer; John Dalton, the originator of the atomic theory; and Troughton, the philosophical instrument maker. Dugald Stewart first discovered the defect on hearing a member of his family admire the contrast of colour between the leaves and fruit of a Siberian crab-tree, while he could see no difference between them, except in point of form and size. John Dalton could not distinguish blue from crimson, and he could only see two colours, blue and yellow, in the prismatic spectrum. Troughton could see no difference between dark crimson, bright orange, and yellow—in fact, he could only distinguish blue from yellow.

In an article on this subject, published in the Magasin Pittoresque for 1846, a Swiss physician gives some interesting examples, which are worth repeating. In the solar spectrum obtained by passing a ray of light through a triangular prism, and which is composed of the following colours,—red, orange, yellow, green, blue, indigo, and violet,—Dalton could only see yellow, blue, and violet. Rose-colour by day appeared to him a pale blue, but at night it seemed to take an orange hue. By day crimson seemed to be dirty blue, and red cloth dark blue. Dr. Whewell having asked him one day to describe the colour of the doctor’s scarlet gown, Dalton pointed to the trees around them, and declared he could distinguish no difference in their colour; and one day having dropped a stick of red sealing-wax in the grass, he had the greatest difficulty in finding it again. Since Dalton’s time over five hundred distinctly marked instances of this imperfection have been noticed, and Professor Prévost, of Geneva, has named it Daltonism, an extremely unphilosophical piece of pathological nomenclature, which has unfortunately received the sanction of too many great physiologists to be abolished. Blindness might just as well be called Homerism or Miltonism.

Colour-blindness is much more frequent than is generally supposed, for those who are afflicted with it are mostly ignorant of the defect, and frequently practise trades or professions in which perfect sensibility to the different hues of colour is quite indispensable. An instance of this occurred some time since in the case of an engine-driver, who allowed his engine to run into a luggage train, through not noticing the red danger signal. At his examination it was proved that he was colour-blind, and could not distinguish red from green. Partial colour-blindness is, no doubt, the cause of the frequent disputes that we hear about the tints of certain objects; to say nothing of the glaring instances of bad taste in the arrangement of colour that are now-a-days so common. Out of forty boys at a school at Berlin who were examined by Leebech, he found five who were quite confused in their notions of colour, and could not distinguish between ordinary shades of the same hue. This affliction is in many cases hereditary, descending from father to son. It is singular that instances of colour-blindness are much more common amongst men than amongst women, for out of over five hundred cases there were only four in which females were the sufferers. It seems also that persons with grey eyes are more frequently colour-blind than those whose eyes are blue or brown. To the list of great men who were colour-blind, we must not forget to add the celebrated Italian historian, Sismondi.

Physiologists consider that there are two kinds of colour-blindness,—one where only two colours are seen, the other where more than two are perceptible. Daubeny Turberville, an oculist of Salisbury, mentions a case of the former, in which a young girl, like the Maryport shoemaker mentioned by Brewster, could only distinguish between black and white, everything between the two being of different shades of grey. This girl, singularly enough, could see to read in twilight a quarter of an hour after her companions. This sharpness of sight appears to be not at all uncommon amongst those who are colour-blind. Spurzheim mentions the ease of a whole family who were afflicted in the same way as Turberville’s patient. All the male members of Troughton’s family were equally incapable of distinguishing any colours but blue and yellow.

The cases of colour-blindness where more than two colours are distinguishable, are much more common. Goethe, the great German poet, who dabbled a great deal in optics, knew two young men who, although they possessed powerful sight, and could distinguish between white, black, grey, yellow, and orange, were at a loss when the shades between dark red and rose-colour were in question. A piece of dried carmine appeared bright red to them, and a faint carmine hue on a white shell, and a rose-leaf, light blue; the leaves of trees and grass appeared yellow, and they confounded rose-colour, blue, and violet together. Goethe supposed them to be incapable of perceiving blue and its several hues, and called their defect by a high sounding Greek name, akyanoblepsy, or blue-blindness. Péclet mentions two other persons, also brothers, who likewise were incapable of distinguishing between blue, violet, and rose-colour. Like Professor Whewell, they confounded the dull scarlet of the trousers of the French infantry with the leaves of the trees. Yellow appeared to them more brilliant than any other colour. Doctor Sommer and his brother could not distinguish between red and its derivatives and other colours; they could only distinguish between yellow, blue, white, and black. Doctor Nicholl mentions a child that could only see red, yellow, and blue, in the spectrum. It could distinguish green, but called it brown when it was dark, and pink when it was pale. The same physiologist knew a man who called red green, and brown dark green. A young lady who was an amateur artist, could not perceive a piece of scarlet cloth hanging on a hedge that was close to her, although others could see it plainly half a mile off. One day she gathered, as a great curiosity, a lichen which she supposed to be of a bright scarlet hue, but which was in reality of a beautiful green. Another time she could see no difference between carmine and prussian blue. A gardener living at Clydesdale, who began life as a weaver, was compelled to give up his first trade because in daylight he confounded all light colours; yellow and its varieties he could distinguish perfectly, but he was incapable of seeing any difference between red, blue, pink, brown, and white. Another man, who was a silk-weaver, had to change his trade, because he could not distinguish between red, pink, and sky blue. A Genevese artist whom circumstances compelled to paint a portrait by candle-light, used yellow for pink in laying on his flesh tints, with a pleasing result that may be readily imagined. In fact, the instances of colour-blindness mentioned by physiological writers are almost innumerable, and I should only weary my readers if I related all the authentic cases of this singular affliction. One instance, however, which was very carefully observed by Wartmann, a distinguished German oculist, merits our attention. The afflicted person, whom Wartmann speaks of as D., was thirty-three years old. Those of his brothers and sisters whose hair was fair suffered from the same infirmity, but those whose hair was dark were exempt from it. Like so many others who are colour-blind, he could not distinguish between cherries and their leaves, and confounded a sea-green piece of paper with a scarlet ribbon placed near it. A rose of the ordinary hue appeared greenish-blue. Being anxious to see if reflected, refracted, and polarized light exercised a different action on his retina, Wartmann tried him first with the prismatic spectrum, but he could only distinguish four colours,—blue, green, yellow, and red. He could distinguish perfectly the peculiar black lines seen crossing the spectrum in certain places, and known by the name of Fraunhofer’s lines. He then placed in his hands thirty-seven pieces of differently coloured glass, but he could only distinguish four varieties. The colours produced by polarized light seemed to give the patient quite as much trouble as those produced in the ordinary way. Chocolate brown appeared reddish brown; purple, dark blue; and violet, a dirty blue. When colours were illuminated by sunlight, they seemed to him to be redder than usual, even green and blue appearing red.

In considering cases of colour-blindness, it is very difficult not to be misled into using wrong terms, as applied to colour, for we have no possible means of knowing what colour it is that is really seen by the patient. Thus, for instance, Dr. Whewell could not distinguish between red and green. But what colour did he really see? Did he see the leaves and cherries both red or both green, or was it some colour between the two that was impressed upon his retina? Again, great care must be exercised in placing implicit reliance on the statement of persons who are colour-blind, for we must recollect that their only means of conveying the results of what they experience is by the use of an organ that is confessedly defective, and which is quite likely to deceive them, and us too, without their being parties to the deception.

The cause of colour-blindness is completely unknown; philosophers and physiologists are still in the realms of hypothesis concerning this peculiar optical defect. As yet, the most careful observation has failed to detect any difference between the eyes of those who are colour-blind, and the eyes of ordinary persons, that could in any way account for this singular affection of the sense of sight.

CHAPTER VI.
ILLUSIONS CAUSED BY LIGHT ITSELF.

When playing about the Christmas fire, children frequently amuse themselves by whirling round and round a piece of wood, one end of which they have previously lighted and blown out. In proportion as the movement becomes more rapid, the path of the red-hot end becomes more and more connected, until at last a burning ring is formed, in every part of which the shining charcoal appears to be at the same time. The only way of accounting for this illusion is by supposing that the image formed by the burning stick upon the retina remains there for an appreciable period, the impression made by it at one part of its journey remaining until it returns to its former position. The power possessed by the retina of retaining impressions explains a large number of illusions of the same kind. The chord of a musical instrument, for instance, when struck, appears to occupy a longer space during the time it vibrates, than when it is at rest. A rapidly revolving wheel appears almost solid on account of the combined images of the spokes seeming to unite into one homogeneous mass.

The persistence of luminous impressions upon the retina has given rise to the invention of a number of well-known optical toys, amongst which may be mentioned the phenakistiscope, the thaumatrope, the phantascope, and many others.

Fig. 3.—The Phenakistiscope.

The phenakistiscope may be described ([figs. 3] and [4]) as consisting of an iron pin a b turning easily on its axis, and passing through two holes in a brass rod t g, bent twice at right angles. Attached to one end of the pin is a disc of cardboard, divided into several equal sectors, and pierced near its circumference with as many similar sized rectangular holes ([fig. 4]). In each sector the same scene is represented, with this difference, that the movements of the objects are so arranged as to be progressive from one extreme to the other. The disc being fastened to the pin a b ([fig. 3]) by the screw v, with the figures facing outwards towards a, the whole apparatus is held before a looking-glass by the handle m. If the disc be now rotated by the button b, and the eye placed opposite one of the square holes in the card, the figures on the disc will appear to move more or less quickly according to the rate at which it is rotated. The three bricklayers in [fig. 4] will be seen to pass their bricks from one to the other with perfect regularity if the drawing has been made carefully. Numberless other designs may be made for this little instrument, such as a windmill in full sail, a man working a pump, a conjurer swallowing knives—in fact, any scene with objects in motion may be drawn, and will cause infinite amusement for the long winter evenings.

The time during which the impression of any object remains upon the retina appears to be in direct proportion to its brilliancy. For a burning coal it is stated to be about the tenth of a second; consequently, if the stick mentioned at the beginning of the chapter is rotated ten times in a second, a continuous luminous ring will appear to be formed. That the time necessary for producing a distinct impression varies with the brilliancy of the object, may be readily guessed from the fact that an electric spark is perfectly visible, although its duration can hardly be measured, while a cannon-ball in flight is only perceptible to the practised eye of the artilleryman, owing to its reflecting only a small quantity of very diffused light.

Fig. 4.—Disc of the Phenakistiscope.

The second instrument, the thaumatrope, is constructed on the same principle. It consists of a certain number of circular discs of card three or four inches across, which are capable of being turned on their axes with great rapidity by means of the finger and thumb and a couple of silk threads fixed at opposite sides of their circumference. On each of these discs a design is painted, one-half appearing on one side, and the other half on the other, in such a manner that the two parts form a single picture. You may have, for instance, Harlequin on one side and Columbine on the other, but on turning the card you will see them together. The body of a Turk may be drawn on one side and his head on the other, and, by rotating the card, the head suddenly finds a pair of shoulders to fit it. A sentence may be divided in the same way, or the words, or even the letters, may be divided between the opposite sides of the card: in fact, like the phenakistiscope, the designs applicable to this little instrument are endless.

The third of these instruments, the phantascope, is constructed in accordance with the peculiar power possessed by the eyes of adapting themselves to the distance of the objects they are looking at. Everybody must have noticed that in order to see objects plainly that are placed at different distances we insensibly alter the position and focus of the eyes, and that, consequently, objects even in the same plane as those we are looking at are not perceived by us until something calls our attention to them, and causes us to alter the position and focus of our eyes and fix our gaze on them. For instance, in looking at a canary in a cage, we have but a confused idea of the wires, which we will suppose to be midway between the bird and the observer. But if anything attracts our attention to the wires we lose sight of the bird, or at any rate see it only as a confused mass. If this experiment is made with care, it will be perceived that the object seen confusedly is always double,—a fact that may be verified by interposing the finger between the eyes and any object. When we look at the finger, the distant object will seem to be doubled; if we look at the object, it is the finger that undergoes duplication.

We know by experience that when we look at an object and press one of the eyeballs slightly with the finger, the image of it becomes doubled. The explanation of this phenomenon is not very easy, but it is generally supposed that in the case of ordinary vision the two eyes produce the sensation of a single image in consequence of the two impressions being formed at corresponding parts of each retina, and that habit causes us to see only a single object in such a case. But when the eyes are so disposed as to be capable of seeing distant objects distinctly, the two images formed by a near object are no longer found in the corresponding portions of each retina, and so produce the sensation of double vision. The same thing happens when either of the eyes is momentarily displaced.

These phenomena have given rise to the construction of a very simple instrument, the phantascope, with which many interesting experiments may be performed, and which was invented some years since by Dr. Lake, an eminent physician of New York.

In the middle of one of the edges of a thin piece of wood, say six inches or a foot in length, which serves as a base for the instrument, is fixed a rod fourteen or sixteen inches long, upon which slide a couple of ferules capable of being fixed at any height by means of thumb-screws. Each of these ferules holds a piece of cardboard five or six inches long, and of any convenient breadth, in a horizontal position. The upper card is pierced in a longitudinal direction with a slit rather less than a quarter of an inch broad, and about three inches long; that is to say, a little wider than the distance between the centres of two eyes. The second card has a similar slit of the same length, and corresponding vertically with the one above it; the width, however, in this instance being only about the eighth of an inch. In addition, the lower card should be marked with a fine line drawn across the centre, which we shall call the index.

Things being thus arranged, if we place two similar objects—two A’s, for instance—upon the wooden stage of the instrument, about three inches apart, and look at them through the two slits, we shall see them as under ordinary circumstances; but on fixing our eyes intently on the index of the lower card, and gradually raising it, we shall see the two A’s become double, the two images of each letter separating themselves more and more the nearer the lower card approaches the upper one, until the last two of the images will coalesce, and appear to be placed on the lower cardboard, the other two remaining in their proper place. The eyes must be kept firmly fixed upon the index, otherwise the illusion disappears immediately, and two A’s only are seen in their true position on the base of the instrument. This is an instance of the production of an image in a place where it certainly does not exist. This illusion is seen best when the upper screen is about ten inches from the object, the lower screen being just half-way between; but, as in most of these cases, the distances will differ according to the focus of the observer’s eyes. The proper distances once being found, the experiment may be varied in a hundred different ways. For example, instead of two letters and a line we may have two flowers on the stage, and the figure of a flower-pot on the intermediate screen. If the two flowers are painted different colours, they will unite and form a mixed tint. Thus a red and yellow flower will give an orange image, a blue and yellow a green image, and so on. A perpendicular stroke and a horizontal one will give a cross. A few experiments with this little instrument will throw a light upon many of the obscurer points that exist amongst the phenomena of vision, and will show conclusively that the two eyes rarely see in the same manner, and that it is sometimes one, and sometimes the other, that sees most distinctly. A couple of pieces of cardboard, pierced with suitable slits and held in the hand may be substituted for the apparatus above described, but of course they will be much more difficult to use, and will give less satisfactory results.

CHAPTER VII.
THE INFLUENCE OF THE IMAGINATION.

The above facts show plainly that optical illusions find their source in the very mechanism of the organs of sight, and that without going farther than the eye itself we may discover numberless examples of these phenomena. We shall presently bring before our readers the innumerable means devised by art for deceiving the sense of sight and impressing us with sensations that are purely imaginary. But before describing these numerous pieces of apparatus we must still remain for a short time within the domain of man’s faculties, and describe some of the illusions that we are subjected to by those powers of the imagination that are supposed to hold in check the five senses of the body. Our imagination, however, plays us as many tricks as our eyes, and, like them, is alternately false and true. Touch, taste, smell, hearing, and sight, are all supposed to be under its powerful influence for good or evil; but they are all deceived by it in turn, more especially the sense of sight, which we generally boast of as being the most trustworthy of them all. Were we to describe all the labyrinths into which our imagination is continually leading us, we might easily extend this little volume to one of treble the size. But our purpose is not so much to write a history of all the hallucinations to which the imagination is subject, but to cull from those already existing the most interesting instances in which this great faculty is alternately the victim and the tyrant of the sense of sight.

Amongst many works on this subject we may cite that of Brière de Boismont on “Hallucinations, Apparitions, Visions, &c.,” from which we shall draw largely in the following pages. The examples we shall give will be those only in which the victims of the hallucination were in the full enjoyment of their mental faculties, and could healthily analyze the sensations and impressions to which they were subjected.

One of the first of these bears upon those diseases of the eye to which allusion was made in [Chapter IV]. Towards the end of 1833, a poor washerwoman who was tormented grievously with rheumatic pains gave up her business, and took to sewing for her livelihood. Being but little accustomed to this kind of work, she was compelled to sit over her needle late at night in order to save herself from starving. The unwonted strain upon the eyes soon brought on ophthalmia, which speedily became chronic. Nevertheless, she continued her work, and fell a prey to diplopia, or double sight in each eye. Instead of a single needle and thread, she saw four continually at work, everything else about her being similarly multiplied. At first she took no notice of the singular illusion, but at last both imagination and sight joined arms against the judgment, and the poor creature imagined that Providence had taken pity on her forlorn condition, and had worked a miracle in her favour by bestowing on her four pair of hands in order that she might do four times her usual amount of work.

The following is another instance of the passage of illusion into hallucination. A man fifty-two years old, of a plethoric constitution, after having suffered from a defect in his visual functions that caused him to see objects sometimes double, and at others upside down, suddenly showed signs of cerebral congestion, and threatened apoplexy. By proper treatment, however, he was saved for a time from the latter catastrophe, but he became permanently afflicted with strabismus, or squinting, and he suffered from a singular hallucination. His eyelids would contract, and his eyeballs would roll from side to side at more or less distant intervals. On these occasions he imagined he saw the figures of different persons that he knew moving about, and would even follow them outside his door into the other rooms of the house. He was perfectly aware that these appearances were merely the effect of the imagination, but this did not in any way detract from their appearance of reality. The man afterwards died from an attack of apoplexy.

The following examples are also cases of singular optical deception, some of them being so extraordinary as to trench upon the supernatural, and in the days of ignorance would have given those who were their victims the character of unearthly personages.

A certain English painter, who in some sort inherited the palette of Sir Joshua Reynolds, and believed himself superior in many respects to the great master, used to boast that in one year he painted over three hundred portraits, large and small. This fact seemed to Wigan a physical impossibility, and he questioned him closely as to the secret of his astonishing rapidity of execution, for he never required more than one sitting from his patrons. Wigan states that he saw him paint a miniature of a well-known personage in eight hours, which was incomparable in its fidelity to nature and finished execution. Wigan asked him to give him some details of the method he adopted, and he gave him the following answer: “When a sitter presents himself, I look at him attentively for half an hour, sketching the outlines of his features on my canvass during the time. I have no occasion for a longer sitting, and I pass on to some one else. When I wish to continue the first portrait, I take the sitter in my imagination, and I seat him in the chair, where I see him as distinctly as if he were really there, and I can even heighten a tint, or soften down a clumsy form at will, without altering the likeness. I look from time to time at the imaginary figure, and I go on painting. I stop now and then to examine his position, absolutely as if the original were before me; for every time I look towards the chair I see the sitter. This method of proceeding has rendered me very popular; and as I have always succeeded in catching the likeness of my patrons, they have been simply enchanted at my sparing them the tedious sittings exacted by other painters. Little by little I have begun to lose the distinction between the real and imaginary sitter, and I have often maintained stoutly that my patrons had already sat to me on the previous day. At last I became convinced that it was the real sitters that I saw, and thenceforth all became confusion. I suppose my friends took alarm at my hallucinations, for I remember nothing of what happened during the thirty years that I remained in the madhouse. This long period has left no trace on my memory, except indeed the last six months of my confinement. It seems to me, however, that when my friends talk of having visited me I have some vague recollection of the fact; but it is a subject that I do not care to pursue.”

The most remarkable feature of the case is, that this artist after a lapse of thirty years resumed his pencil, and painted almost as well as when he was forced by madness to abandon his art.

This faculty of being able to evoke shadows, with which to people one’s solitude, may be carried so far as to transform real persons into phantoms. Hyacinth Langlois, a distinguished artist, living at Rouen, tells us that Talma, with whom he was extremely intimate, confided to him that, whenever he went upon the stage, he had the power, by mere force of will, to cause the clothes and flesh of his numerous auditory to disappear, and become transformed from living beings into so many skeletons. When his imagination had peopled the house with these singular phantoms, the emotion he felt was so great that it gave his dramatic powers still greater force, and enabled him to produce the wonderful effects that have made his name so famous.

Wigan says, that he once knew a most intelligent and amiable man, who could at will evoke his own image. He often laughed at seeing his second self standing before him, the phantom appearing to laugh as heartily as himself. This illusion was for a long time a matter of amusement to him, but at last he became persuaded that he was haunted by his own double. His second self appeared to hold arguments with him continually, and beat him frequently on various points of dispute, a matter which mortified him excessively, as he was rather proud of his powers of reasoning. This gentleman, although always considered as being somewhat eccentric, was never put under the slightest restraint, and at last the creature of his imagination so tormented him, that he resolved not to live through another year. He consequently paid all his debts, arranged his affairs, and waited pistol in hand until the clock struck twelve on the 31st of December, and then deliberately blew out his brains.

In Abercromby on the Mind we read an account of the observations made by a gentleman who was the victim of illusions during the whole of a pretty long life. If he met a friend in the street, he was unable to tell at first whether he saw a real human being or only a phantom. By close examination he could detect a difference between the real person and the creature of his imagination, the features of the former being sharper and more defined than those of the phantom; but in general he was obliged to test the reality of the figure he saw by the senses of touch and hearing. He was able, by concentrating his thoughts upon the appearance of any friend, to call up his image; a power which extended even to scenes that he had witnessed. Although he could produce these hallucinations at will, he was powerless in making them disappear; and when once he succeeded in calling forth these creatures of his imagination, he never could tell how long the delusion would last. This gentleman was in the prime of life, a good man of business, and otherwise in a perfect state of mental and bodily health. A member of his family possessed the same faculty, but in a minor degree.

In 1806, General Rapp, when returning from the siege of Dantzic, having occasion to speak to the Emperor Napoleon, walked into his private room without being announced, and found him in such a profound state of abstraction, that he remained for some time unperceived by his imperial master. The General, seeing him thus perfectly motionless, fancied he must be ill, and purposely made a slight noise. Napoleon instantly turned his head, seized the General by the arm, and pointing upwards, exclaimed, “Do you see it up there?” The General, hardly knowing what to say, remained silent; but the Emperor repeated his question, and he was obliged to reply, that he saw nothing. “What,” said the Emperor, “you don’t see anything? You don’t see my star shining before your eyes?” And becoming more and more animated, he went on to say, that the mysterious visitor had never abandoned him, that he saw it throughout all his great battles, that it always led him onward, and that he was never happy but when he was gazing at it.

That such hallucinations have no real existence as far as the eye goes, is proved by the fact of many people who have lost their sight, being subject to them. It is hardly to be wondered at that those who by accident have been deprived of their sight, should wish so ardently to see once more the persons and sights they have taken pleasure in, that they should at last create for themselves illusions of this character. The same thing has frequently occurred with those whose sight is more or less weak. An old man of eighty, who was purblind, never sat down to a table during the last years of his life, without seeing around him a number of his friends who had long been dead, dressed in the costume of fifty years before. This old man had but one eye, which was extremely weak, and wore a pair of green preservers, in the glass of which he continually saw his own face reflected.

Doctor Dewar, of Stirling, mentioned to Abercromby a very remarkable instance of this species of hallucination. The patient, who was quite blind, never walked in the street without seeing a little old woman hobbling on before him and leaning on a stick. This apparition always disappeared when he entered his house.

Similar illusions frequently happen to every one, even the most healthy amongst us, but a little consideration soon puts them to flight. It would be useless to mention the numberless cases in which a square tower has appeared round, or where the landscape has suddenly seemed to recede from the sight. Such illusions as these have been long well known, and appreciated at their proper value; but there are others whose true cause has remained a mystery, until explained by the progress of science, such as the Spectre of the Brocken, the Fata Morgana, and the mirage.

Analogous appearances have been seen in Westmoreland and other mountainous districts, the inhabitants imagining that the air was full of troops of cavalry, and whole armies even; such illusions resulting simply from the shadows of men and horses passing over an opposite mountain being thrown on the fog.

A vast number of different circumstances give rise to these illusions, such as a strong impression, or the recollection of some striking event, which may easily cause them, by the association of ideas. Wigan relates, that being at a soirée held at the house of M. Bellart, a few days after the execution of Marshal Ney, the groom of the chamber, instead of calling out the name of M. Maréchal aîné (M. Maréchal, senior), announced the arrival of M. le Maréchal Ney. A shudder passed through the company, and many of them declared, that for an instant they saw the face and figure of the dead man in place of those of his involuntary representative.

When the mind is thus prepared, the most familiar objects are transformed into phantoms. Ellis relates an anecdote of this kind, which he heard from an eye-witness, who was a ship’s captain of Newcastle-on-Tyne. During a voyage that he made, the ship’s cook died. Some days after the funeral, the chief mate came running to him in a great fright, with the news that the ship’s cook was walking on the water, astern of the vessel, and that all the crew were on deck looking at him. The captain, who was angry at being disturbed with so nonsensical a tale, answered sharply, that they had better put the ship about and race the ghost to Newcastle. His curiosity, however, was presently aroused, and he went upon deck and looked at the spectre. He frankly avowed that for some moments he saw what really appeared to be his old shipmate, just as he knew him in life, with his walk, clothes, cap and figure perfectly resembling those of the dead man. The panic became general, and every one was struck motionless for a time. He had the presence of mind, however, to seize the helm and put the ship about, when as they neared the ghost, they found the absurd cause of their fright was a broken mast from some wreck, which was floating after them in an upright position. If the captain had not boldly sailed up to the supposed ghost, the story of the dead cook walking upon the water would have continued to this day to terrify half the good inhabitants of Newcastle.

Such facts as these are innumerable, and we shall mention a few more which will explain a host of stories found in various ancient and modern authors.

Ajax was so angry at the arms of Achilles being awarded to Ulysses, that he became furious, and, seeing a herd of pigs, drew his sword and fell upon them, taking them for Greeks. He next seized a couple of them and beat them cruelly, loading them at the same time with insults, imagining one of them to be Agamemnon, his judge, and the other Ulysses, his enemy. When he came to himself, he was so ashamed at what he had done, that he stabbed himself with his sword.

Theodoric, blinded by jealousy and yielding to the base solicitations of his courtiers, ordered that Symmachus, one of the most upright men of his time, should be put to death. The cruel order had hardly been executed, when the king was seized with remorse, and bitterly reproached himself with his crime. One day a new kind of fish was put upon the table, when the king suddenly cried out that he saw in the head of the fish the absolute resemblance of that of his victim. This vision had the effect of plunging the king into a state of melancholy that lasted his whole life.

Bessus once, when surrounded by his guests and giving himself up to the enjoyment of the feast, ceased suddenly to listen to the flattering speeches of his courtiers. He apparently listened with great attention to some sound that was heard by no one else, and suddenly leaping from his couch, mad with rage, he seized his sword and rushing at a swallow’s nest that was near, beat it down, killing the poor birds inside it, crying out that these insolent birds dared to reproach him with the murder of his father. Surprised at such a sight, his courtiers gradually disappeared, and it became known some time afterwards that Bessus was really guilty, and that the senseless action he had performed simply resulted from the voice of conscience.

The illusions of sight and hearing are often found to take an epidemic form, and historians relate an immense number of anecdotes bearing on this particular phase of self-delusion. One of the commonest of them is that which transforms the clouds into armies and figures of all kinds. Religious prejudices, optical phenomena, physical laws that are still unknown, dangerous fevers, derangements of the brain, afford a natural explanation of these hallucinations.

We have borrowed most of these examples from Brière de Boismont’s works, for the special purpose of showing how easy it is to deceive the imagination, and to demonstrate the facility with which the sense of sight is led astray without the intervention of complicated apparatus. In addition, we may quote instances from Brewster, showing the ease with which the imagination enables us to see distinct forms in a confused mass of flames, or in a number of shadows superposed upon each other. This great philosopher gives us an anecdote of Peter Heamann, a Swedish pirate and murderer. One day that his crew were repairing some unimportant portion of the ship, after having pitched the place well he took the brush in order to tar the other parts of the vessel, which were much in want of such treatment; but as soon as he spread the pitch over the timbers of the ship, he was thunderstruck at seeing apparently reflected in its shining surface the image of a gallows with a headless man beneath. The head belonging to the body was lying before it, and the body itself was depicted with every limb—legs, thighs, and arms—perfect. He frequently told his crew of these illusions, adding that it was evidently a prediction of the fate in store for them. He was often in such a state of terror, that on calm days he would drop down into the hold and wrap himself up in a spare sail in order not to catch sight of the horrible image that he constantly saw in the shining surface of the tar.

The imagination really seems to create for itself a sort of mental visual organ which is in intimate relation with that of the body, and which often takes its place so efficiently—as in the case of dreams—that the mind is utterly unable to perceive the substitution. It is on account of this that practical opticians are so unsparing in their endeavours to predispose their spectators to being deceived.

When both the body and mind are healthy, the relative intensity of the two kinds of impressions is very unequally divided, mental images being more evanescent and comparatively weak, and with persons of ordinary temperament incapable of effacing or disturbing the reflections of visible objects. The affairs of life could not go on if the memory introduced amongst them brilliant representations of the past in the midst of ordinary domestic scenes or the objects familiar to us. We may account for this by supposing that the set of nerves which carries the efforts of the memory to the brain cannot execute their functions at the same time as those which take cognizance of the images reflected on the retina. In other words, the mind cannot accomplish two separate functions at one and the same time, and the mere act of directing the attention to one class of subjects causes all others to become instantly imperceptible. The exercise of the mind in these instances is, however, so rapid that the alternate appearance and disappearance of the two different impressions is completely unnoticed. Thus, for instance, while looking at the dome of St. Paul’s, if our memory suddenly evokes the image of some other object, Mont Blanc for instance, the picture of the cathedral, although still depicted on our retina, is momentarily effaced by the effort of the will, although we may not change the position of our eyes during the time. While the memory continues to dwell on the picture it has called up, it is seen with sufficient distinctness, although its details may be somewhat misty and its colours confused; but as soon as the wish to see it passes away the whole disappears, and the cathedral is seen in all its former distinctness.

In darkness and solitude, when surrounding objects produce no images that can interfere with those of the mind, these latter are more lively and distinct: and when in addition we are half asleep and half awake, the intensity of mental impressions approaches that of visible objects. In the case of persons of studious habits who are continually employed in mental effort, these images are more distinct than with those who follow the ordinary avocations of life, and during their working hours rarely see the objects round them. The earnest thinker, absorbed by meditation, is in a manner deprived for the time of the use of his senses. His children and servants pass in and out of his study without his seeing them, they speak to him without his hearing them and they may even try to rouse him from his reverie without success; and yet his eyes, ears, and nerves received the impression of light, sound, and touch. In such instances, the mind of the philosopher is voluntarily occupied in following out an idea which interests him profoundly; but even the most unlearned and thoughtless of us sees the images of dead or absent friends with his mind’s eye, or even fantastic figures which have nothing to do with the train of thought he may be pursuing. It is with these involuntary apparitions as with spectres of the imagination: although they are intimately connected with some thought that has passed through our mind unperceived, it is impossible to trace a single link of the chain connecting them together.

PART II.

THE LAWS OF LIGHT.

CHAPTER I.
WHAT IS LIGHT?

Everybody knows the effects of the action of light, without, however, understanding precisely what constitutes light itself. Any formal definition would rather puzzle than help the student; we must therefore content ourselves with saying that light is that effect of force which causes us to perceive external objects.

A man who was blind from his birth, and upon whom the operation for cataract had been successfully performed, had accustomed himself for a long time to imagine the nature of those unknown phenomena that his affliction had prevented him from observing. He had arranged in his mind the various definitions that had been given to him as to the nature of light, and having combined them, he fancied he had acquired some notion of what the sense of vision really meant. But what was the astonishment of the surgeon who had restored to him his fifth sense, when he asked him to give his opinion upon the effects of light, to see him take up a lump of sugar and reply that it was under that form that he had imagined it to himself.

As for us who have the happiness of possessing the sense of sight, we know this mysterious agent more by the enjoyment that we have derived from it, than from any analysis we have made of its nature. It is an endless chain that connects us with the entire universe; a bond that laughs at distance and spans the abysses of space. By means of light we can appreciate the beauties of hue and form, and by its power we touch as it were the inaccessible. It constitutes the most intimate connexion between ourselves and external objects—a connexion that seems even to alter our temper, disposition, and character, according to the variations of its intensity. The dull and foggy days of winter, those days when sleet and rain struggle in the atmosphere, spread like a veil over us, and throw a shadow upon our life. The return of the bright spring sun, the reappearance of light and blue sky, on the contrary, open up our hearts and minds, gay nature enchants us once more, and a feeling of fresh happiness prepares us for the coming glories of the newly risen year.

This intimate connexion between the light of heaven and the human mind, hallowed as it is by our desire to rise towards the Source of all light, might be made the subject of many eloquent pages; and it would be an interesting and useful task to show the gradual progress of mankind from those ancient people who trembled at the approach of darkness, and who fervently saluted the dawn with prayers and praises, down to the philosophers of the present age, who investigate its effects with so much reverential joy. But we must cease paying any more attention to the superficial action of this marvellous force which in these latter days has become, in the hands of man, the source of so many illusions and the origin of a complete world of rich and brilliant pictures, but which after all only exist in the imagination.

It was believed for a long time that light was a compact mass of tiny particles emitted by luminous bodies, which struck our eyes and so produced the phenomenon of vision. These particles or molecules were naturally thought to be extremely minute, and the objects illuminated by them were supposed to throw them off as if they were endowed with elasticity. Under this hypothesis, light was a material body. The illustrious Newton was the first propagator of this theory; the last was M. Biot, a French philosopher, lately dead.

The undulatory theory has now-a-days completely superseded the corpuscular hypothesis. It was first started about the year 1660 by the Dutch philosopher Huyghens, who has left behind him numerous treatises on optics, and the properties of light, as well as a curious account of the inhabitants of the other members of the solar system, including a minute description of the various planetary manners and customs. At the beginning of the present century, Fresnel showed, by the most brilliant discoveries the superiority of this theory, and shortly after Arago confirmed him in his demonstrations. According to the undulatory hypothesis, light is not a mass of molecules emitted by a luminous body, but simply the vibration of an elastic fluid which is conceived to fill the whole of space. A comparative example may assist you in understanding this theory more clearly. If you throw a stone into a smooth piece of water, there will form around the point where the stone fell, a series of circular undulations, starting from the centre and gradually enlarging themselves. If a loud noise is suddenly heard, the same effect is produced round the point from whence the sound proceeds. A series of waves are formed which spread not only horizontally, as on the surface of the water disturbed by the stone, but in every direction. In fact, in the case of sounds, the waves are so many gradually increasing spheres. In the case of light, when a luminous body is placed in space, the ether which surrounds it is thrown into a state of vibration, and the motion is immediately propagated in all directions, with extreme velocity. It is these undulations that produce upon our eyes the sensation of light. We may therefore say that light, like sound, is movement, while darkness, like silence, is absolute rest.

Many people still believe that light is propagated instantaneously, and cannot bring themselves to imagine that we do not see a flame the moment we light it, but only an instant after. I have myself spoken to well-educated people possessed of good judgment and a certain amount of elementary knowledge, who could never bring themselves to believe that we see the stars, not as they now exist, but as they appeared at the particular moment when the luminous wave by which we are enabled to perceive them left their surface, and which only reaches us after travelling through space a certain number of years, days, or hours, according to their distance. It is extremely useful and interesting to form a correct idea upon the way in which light is propagated.

The determination of the prodigious quickness with which the waves of light move through space, says Arago, is undoubtedly one of the happiest results of modern astronomy. The ancients believed that it moved with infinite velocity, and their view of the subject was not, like so many of the questions relating to physics, a mere opinion without proof; for Aristotle, in mentioning it, brings forward the apparently instantaneous transmission of daylight. This notion was disputed by Alhazen, in his Treatise on Optics, but only by meta-physical weapons, which were again opposed by several very worthless arguments, by his commentator, Porta, although he admitted the immateriality of light. Galileo seems to have been the first amongst modern philosophers who endeavoured to determine the velocity of light by experiment. In the first of his dialogues, Delle Scienze Nuove, he announces by the mouth of Salviati, one of the speakers present, the ingenious means he had employed, and which he thought quite sufficient to solve the question. Two observers with lights were placed at the distance of one mile from each other; one of them extinguished his light, and the other as soon as he perceived it extinguished his. But as the first observer saw the second light disappear the instant he had extinguished his own, Galileo concluded that light was propagated instantaneously through a distance double that which separated the two observers. Certain analogous experiments that were made by the members of the Academy Del Cimento, but at three times the distance, led to precisely the same conclusions.

These attempted proofs seem at first sight to be absurd, when we think of the vastness of the problem to be solved; but we must judge these experiments with less severity, when we consider that almost at the same epoch, men of such well-deserved repute as Lord Bacon believed that the velocity of light, like that of sound, was sensibly altered by the force and direction of the wind.

Descartes, whose theories upon light had so much analogy with those known under the name of the undulatory hypothesis, believed that light was transmitted instantaneously throughout any distance, and endeavours to prove his position by proofs that he thought he had obtained whilst observing an eclipse of the moon. It must be acknowledged, however, that his very ingenious train of reasoning proves that whether the transmission of light is instantaneous or not, it is at least too considerable to be determined by experiments made on the earth, like those of Galileo, and which he vainly hoped would have solved the question.

The frequent occultations of the first satellite of Jupiter, the discovery of which was almost consequent upon that of lenses, furnished Römer with the first means of demonstrating that light was propagated by perceptible degrees.

In tracing out the history of human knowledge, says Dr. Lardner, we have frequently to point out with some little surprise, joined to a feeling of profound humility, the important part played by chance in the advancement of science. In searching zealously after mere trifles which, when found, are of no consequence, we frequently lay our hands on inestimable treasures. The frequency of this fact impresses the mind with the notion that some secret and unceasing power exists, in accordance with which human knowledge and science are continually progressing. It is in physical, as in moral philosophy. In our ignorance—like the dog mentioned by Æsop, which, seeing in the water the reflection of the prey it held in its mouth, dropped the substance and tried to seize the shadow—we are continually searching after trifles; but, more fortunate than the animal of whom we have been speaking, the shadow that we try to seize is often transformed into a rich treasure. We can say with every confidence that “the Providence which shapes our ends,” knows our wants better than we do ourselves, and bestows on us the things we ought to have asked for instead of those we have asked for. We shall find a very simple proof of this in the history of the discovery of the velocity of light.

A short time after the invention of the telescope and the consequent discovery of Jupiter’s satellites, Römer, a celebrated Danish astronomer, was engaged in a series of observations, the object of which was to determine the time which one of these bodies took to revolve round its planet. The method employed by Römer was to observe the successive occultations of the satellite, and to notice the interval that elapsed between each of them. But it at last happened that the interval between the two occultations, which was about forty-five hours, became prolonged by periods of 8, 13, and 16 minutes, during that half of the year when the earth was receding from the planet, while it became proportionally cut short during the rest of the year. Römer was struck by a happy idea; he suspected instantly that the moment when he remarked the disappearance of the satellite was not always coincident with the instant when it really took place, but that it sometimes appeared to happen later—that is to say, after an interval of time sufficiently long to allow the light that had left the satellite immediately after its disappearance, to reach the eye of the observer. Hence it became evident that the farther off the earth was from the satellite, the longer was the interval of time between its disappearance and that of the arrival of the last portions of its light upon the earth; but that the moment of the disappearance of the satellite is that of the commencement of the occultation, and that the moment of the arrival of the last portions of light is that when the commencement of the occultation is observed.

It was thus that Römer explained the difference between the calculated and observed time of the occultation, and he saw that he was on the threshold of a great discovery. In a word, he saw that light propagated itself through space with a certain velocity, and that the fact we have just mentioned furnished the precise means of measuring it.

Thus the occultation of the satellite was retarded one second for every 185,000 miles that the earth is distant from Jupiter; the reason being, that a ray of light takes a second to travel this distance, or, in other words, because the velocity of light is at the rate of 185,000 miles per second.

It must be remembered when considering this subject, that in any system of undulations or vibrations, no matter through what medium they are propagated, their movement is simply a change of form, and not a transmission of matter. The waves which spread round a central point when a stone is thrown into the water, give one the idea that the water which forms the wave really moves towards the observer. But it is not so, as may be readily proved by placing on the surface a floating body, which we shall find is but little, if at all, influenced by the undulations of the water. The appearance of rolling waves given on the stage by means of a painted cloth, to which an undulatory motion is given, is an instance of this apparent movement. In the case of the floating body, which would follow the movements of the water, we shall find that wave after wave rolls to the shore, in the same way as the painted marks on the imitation sea keep their place, although the cloth itself undulates. The waves of the sea even appear to the eye to be endowed with a progressive motion, but an instant’s observation will convince us of our error; for if such were the case, every object floating on the ocean would be gradually carried on shore. A vessel floating on the waves is not carried along by them, at least not until it reaches within a few yards of the shore, where the water is really in motion; but out in the open sea a floating body will alternately rise on their crests, and fall into the valleys that separate them. The same effect may be observed with any object floating on the water. If, however, in addition to being in a state of undulation the sea is really in motion from the effects of a current, or from any other cause, the floating object will of course be carried along by it—in fact, the two movements are quite independent of each other, and may take place in similar or contrary directions. It is very important that we should be able to distinguish at an early period the exact difference between true movement and mere undulation; and we must remember that although the waves of light are propagated at the rate of 185,000 miles a second, still there is no transmission of any material substance at this marvellous rate. The same observation applies to sonorous vibrations transmitted through the air.

Thus we are constrained to admit peaceably the truth of the undulatory hypothesis as compared with the corpuscular theory. I say peaceably, because I am forcibly reminded by the contrast I have made between the two theories of an anecdote related of one of the greatest monsters who ever walked this earth, but who was afterwards struck down in the midst of his power by the hand of a weak girl. I allude to the infamous Marat, who one day presented himself at the house of Dr. Charles, a celebrated natural philosopher, of the time of the first French Republic, in order to advance certain notions of his own against the optical principles that Newton has left behind in his Principia, and other works—also, to oppose certain theories connected with electrical science. Dr. Charles, who did not approve of Marat’s wild notions, undertook to convince him of his errors. But instead of discussing the matter peaceably, Marat allowed himself to be carried away by his temper, which was naturally very violent. Every argument advanced by his antagonist seemed to increase his rage, until at last he lost all control over himself, drew his small sword, and rushed upon his opponent. The doctor, who was unarmed, had to exercise all his powers to prevent himself from being wounded, and being much more stoutly built than Marat, he at last succeeded in throwing him down, and wresting his sword from him, which he immediately took care to break. Whether it was the violence of the fall, the shame he felt at being doubly beaten, or the effects of his fit of passion, does not appear, but Marat fainted. Assistance was called, and he was carried home to his house, his offence against all the laws of propriety being forgiven by his more talented and better-tempered adversary.

There are many persons, no doubt, whom we should astonish, and possibly enrage, by asserting positively that we could cause darkness by means of light, that silence could be produced by sound, or cold by heat. These are daring paradoxes, and at first sight appear almost as reasonable as that of Anaxagoras, a Greek philosopher, who asserted that snow was black. But as I hope that most of my readers do not possess the passionate temper of the French tribune, I will confide to them a little secret that will make these paradoxes plain. It is called by natural philosophers the theory of interference.

The experiments connected with this subject are exceedingly difficult to perform, and require the aid of apparatus far beyond the reach of the ordinary student. It is a case where theory and description are much easier than practice.

If a ray of electric light is thrown upon a screen, it is possible to direct another ray upon the same spot in such a manner that they will extinguish each other mutually. The reason of this phenomenon may be understood, if we remember that light is caused by undulatory movement, and that by opposing two series of waves to each other in such a manner that their vibrations coming in contact produce rest, we can easily see how the waves of light of one ray may be stopped by those of a second.

Going back to our illustration of the eddies on a pool of water, it is easy to prove that by throwing a second stone into the water we form another series of undulations; which are mutually destroyed when they encounter each other. It is the same with the peculiar fluid which, existing throughout space, is thrown in a state of undulation by incandescent bodies; by opposing one set of waves to another we obtain rest as a result.

This fact was first observed by Grimaldi in 1665, and Dr. Thomas Young was the first to offer an explanation. Fresnel used it with great success at the beginning of the century to demonstrate the truth of the undulatory theory, by showing that it could not be explained by any other.

CHAPTER II.
THE SOLAR SPECTRUM.

The white light that the glorious orb of day spreads over the face of nature is the original source of all those brilliant and sombre colours with which the works of the Creator are beautified. To the rays of the sun we owe not only the whiteness of the lily, but the scarlet of the field poppy, the modest blue of the timid violet, the splendour of the peacock’s plumage, the cool green of the meadows, and the purple and gold of the distant mountains. For, as we have hinted before, this white light, which seems of itself so destitute of colour, is productive of every hue that the eye of man is capable of appreciating.

It may seem that I am bestowing too much praise upon our own sun; but if you are surprised that I should seek to exalt this brilliant globe of ever-burning fire, I must ask you to recollect, that though the starry heavens are full of suns as vast and important as ours, and possibly affording brilliant colourless light to worlds full of inhabitants, there are others that give forth rays that are far from being white. Some are as green as emeralds, others are as blue as sapphires, while others give out a warm light like a ruby or topaz. The worlds which surround these can only receive light of a certain colour, or at any rate they are restricted to a few shades and hues. Imagine living in a world where everything was always couleur de rose, or in which the inhabitants were continually looking blue! A residence in either of them for a short time would undoubtedly cause us to appreciate the relative value of our own little sun, small as it is in comparison with some of the mighty orbs floating about in space.

The fact that the light of the sun is the source of all the changing hues to be found on the surface of the earth season after season was first discovered by Newton, and his experiments are easily repeated with a very few and inexpensive appliances.

A small round hole is made in the window-shutter of a room, facing the sun, and the pencil of light proceeding from it is allowed to fall upon the surface of a three-sided prism, held in a horizontal position, and placed at a distance of a few inches from the aperture ([fig. 5], Frontispiece). The pencil of light does not pass through the prism as if it were a plate of glass with parallel sides, but in virtue of the laws of refraction, of which we have already spoken, it is turned out of its natural course, and is thrown upon the wall in the direction indicated in the figure. The pencil of light is not only turned aside, but it is also widened out into a band which is truly painted with all the colours of the rainbow, every tone and hue being of the most marvellous brilliancy. This long coloured stripe, which constitutes one of the most beautiful sights that the science of optics can afford us, is known to scientific men by the name of the solar spectrum.

Before going into the causes that produce these colours, let us first examine their number and position. Beginning at the top, we shall find that they run in the following order:—Violet, indigo, blue, green, yellow, orange, red. The red being lowest is called the least refrangible of them all; or, in other words, in passing through the prism it was bent less out of its course than its companions. Violet, being at the top, is of course the most refrangible. The cause of the separation of the colours of white light is consequently only the effect of their individual character. They were, so to speak, so many streams flowing together until an unexpected deviation in their course caused them to separate. This change in the direction of their flow brought out their personal individuality, and they at once became completely disunited.

Every single tint in the prismatic spectrum is simple, and cannot be decomposed. This may be shown by passing any of them through another prism, when it will be found that no change will take place in the colour or size of the pencil. Hence those worlds already spoken of, whose light of day is red, blue, or green, never see any colours but these. ([Fig. 6], Frontispiece).

It is just as easy to reunite the colours into which white light is decomposed, by applying a second prism in a reversed position to the pencil of coloured light, as it is to separate them in the first instance. The method of accomplishing this is shown in [fig. 7], Frontispiece.

Fig. 8.—The Recomposition of Light.

Another experiment in the same direction consists in reuniting the colours by causing them to pass through a double convex lens, behind which is placed a screen of ground glass, or a card ([fig. 8]). By advancing and withdrawing this screen we can easily find the exact spot where the rays reunite, and form a dazzling spot of white light. This point is called the focus, from a Latin word, signifying “fire-place,” a term which will put the student in mind of the frequently repeated experiment of burning a piece of paper with an ordinary magnifying-glass.

Instead of using a lens, you can, if you please, employ a concave mirror, using the ground glass or cardboard screen, as before. The colours reflected by the mirror unite at its focus, and produce a brilliant white spot in just as conclusive a manner as in the other experiment.

Fig. 9.—Recomposition of Light by means of a Concave Mirror.

A fourth experiment, which is somewhat more difficult for the student to accomplish, consists in causing every one of the seven different colours to be reflected from a separate mirror.

The mirrors in this case are concave, and are so mounted as to be capable of being moved in any direction. By directing each of the seven rays, one by one, upon the same point, you may observe the gradual decomposition of the coloured light. The effect obtained by adding the last colour to the mixture is quite magical, the white circle being produced from two brilliantly-coloured spots.

Fig. 10.—Recomposition of Light by means of a number of Mirrors.

A fifth experiment, first devised by Newton, is also within the reach of the student. On a disc of cardboard the centre and border of which have been previously painted black, are pasted seven strips of paper, painted as nearly as possible of the same colour as the components of the spectrum—or if the student is anything of an artist he may paint the disc in imitation of the spectrum, carefully shading off the tints into each other. If the disc be now rapidly rotated the colours will disappear, and a greyish hue will be seen, which will approach more closely to white, the nearer the colours on the disc are to those of the spectrum. This experiment is not precisely the same in principle as the preceding ones, for it is evident that the colours on the disc do not mix, but only the impressions they form upon the retina. We have already said that such impressions remain on the eye for one-tenth of a second or there-abouts; the disc must therefore revolve at least ten times a second, or the effect will not be perceived.

Fig. 11.—Newton’s Disc.

From these experiments it follows that the colours with which all natural substances are clothed, ought not to be looked upon as belonging to them absolutely, but only as a property dependent on the reflection and absorption of light from their surfaces. The leaves of plants, for instance, must not be regarded as being really green in themselves, but as being capable of absorbing certain portions of light, and reflecting others. Grown in the dark, the green substance contained in the plant and its leaves becomes white, and no longer possesses the property of absorbing red light, and reflecting green. A green leaf placed in red light becomes almost black, from its power of absorbing light of that colour; in the blue it reflects a much greater proportion of the coloured ray. A very striking experiment may be performed with a substance known to chemists as the iodide of mercury. If a little of this salt, which is of a brilliant red, be placed in a watch-glass, and heated over a spirit-lamp, it will gradually sublime, and a card held over it will be covered with a number of light yellow crystals. In this case no change of composition has taken place, but simply a change in the power the salt possesses of reflecting some rays and absorbing others. By simply scratching the surface of the card with a pointed piece of wood, the yellow crystals become transformed once more into the red variety; not only this, the transformation gradually spreads, like a red cloud, over the whole of the deposit. There are some other salts known to chemists which possess the property of dichroism, or double colour. The double cyanide of platinum and barium, for instance, appears violet when viewed in one direction, and yellow in another. Change of temperature is often sufficient to change the colour of bodies—white oxide of zinc, for example, becomes bright yellow when heated. Such instances might be supplied ad infinitum, but enough has been said to prove that colour, after all, is only an appearance, and not an essential property of bodies.

We have already spoken of complementary colours, or those which it is necessary to add together in order to produce white light. Blue, for instance, is complementary to orange, red to green, violet to yellow, and vice versa. But it is not by the aid of the palette that this can be proved, for in the case of coloured pigments the arrangement of their atoms interferes in some way with the success of the experiment, and it is only by means of the colours of the spectrum that such recompositions can be effected.

Although most philosophers consider that there are seven colours in the spectrum, there are others who do not admit it, but assert that there are really only three, red, yellow and blue—which by the superposition of their edges produce the intermediate hues of green and orange. Perhaps it would be nearer to the truth to say that the spectrum is composed of an infinite number of colours of different hues.

We have already stated that every one of these colours is indecomposable, and that there are certain worlds illuminated by a single colour only, instead of possessing the infinite number of tints enjoyed by the inhabitants of the solar system. An idea of this effect can easily be gained in a very simple but surprising manner by inserting panes of glass of different colours in the hole of the shutter of a dark room. If the light is yellow, you will find that all those objects that are capable of reflecting yellow light are coloured by it, while those which are bright red or blue become almost black by absorbing the only light present. If we could procure an object which was perfectly complementary in colour to the yellow glass, it would appear perfectly black. The same experiment may be repeated with the other colours. After remaining in this coloured light for some time, if you suddenly pass out into daylight the complementary colour will tinge everything around you.

Instead of using a room into which coloured light only is admitted, lamps burning with a coloured flame may be employed. Brewster mentions the following experiment, which is a very striking one:—Fill a spirit-lamp with alcohol in which has been dissolved as much common salt as the spirit will take up; on being lit it will be found to burn with a livid yellow flame. A room lighted entirely with one or two lamps of this kind will form a laboratory for some very singular experiments. It should, if possible, be hung with pictures in water and oil colours, and the persons present ought to wear nothing but the brightest colours, and the table be ornamented with the gayest of flowers. The room being first lighted with ordinary daylight, the lamps above mentioned should be brought in, and the daylight carefully excluded, when an astonishing metamorphosis will take place. The spectators will be hardly able to recognise each other; the furniture of the room, and every other object contained in it, will reflect but a single colour. The flowers will lose their brilliant tints, the paintings will appear as if they were drawn in Indian ink. The brightest purple, the purest lilac, the richest blue, the liveliest green, will be converted into a monotonous yellow. The same change will take place in the countenances of those present; a livid paleness will spread over their faces, whether young or old, and those who are naturally of an olive complexion will hardly appear changed at all. Every one will laugh at the appearance of his neighbour’s face, without thinking that he is just as great a subject of laughter to them. If, in the midst of the amusement caused by this experiment, the light of day is admitted at one end of the room, the other end being still lighted with the salt-lamp, every one will appear to be half-illuminated with the livid colour which has caused so much surprise, the other portion of their figure and clothes being of the natural hue. One cheek, for instance, will appear animated with its usual brilliancy, while the other will be that of a corpse; one side of a lady’s dress will be brilliant blue or green, as the case may be, the other a colour that it would puzzle an artist to give a name to. The experiment may be varied by admitting the white light through several small holes in the shutter of the room, every luminous spot painting the place where it falls in its natural colours, and the yellow spectators will become spotted with the most singular tints and hues. If a magic lantern is used to throw on the walls of the room and the clothes of the company any luminous figures, such as those of flowers or animals, they will be coloured with these figures in the tint of the wall or fabric upon which they fall, yellowish colours of course escaping the transformation. If nitrate of strontia be substituted for the salt, a crimson tint will be spread over everything. In fact, a lamp prepared in this way will form a source of endless amusement. It is not necessary to use alcohol for the purpose; wood-spirit or methylated alcohol will serve the purpose equally well. If a lamp is not to be had, a few pieces of cotton-wool, tied on wires and dipped in the salted spirit, will do almost as well.

CHAPTER III.
OTHER CAUSES OF COLOUR.

The colours of the spectrum are to the sense of sight what the tones of the gamut are to the sense of hearing. On the one hand, the differences in the lengths of the sonorous waves constitute the variety of note perceptible by the ear; on the other, the differences in the lengths of the luminous waves constitute the variety of colour perceptible by the eye. By and by, we shall learn both the length and rapidity of these vibrations, but it will be as well first to describe the experiments made in this direction by the immortal Newton himself.

Every one has, doubtless, at one period of his life, amused himself with blowing soap-bubbles by means of a tobacco-pipe and a little lather—a sufficiently childish amusement, you will possibly say, but one narrowly connected with the most intricate secrets of the science of optics. These little globes, so fragile that they disappear in a breath, hardly seem worthy of the attention of a thinker, and still less the examination of a philosopher; but it is nevertheless true that Newton made experiments on the colours shown on the surface of these apparently insignificant objects which ended in the most brilliant discoveries, just as on seeing an apple fall he began a train of thought which only terminated in the enunciation of the hypothesis of the earth’s power of gravity.

All transparent substances, whether liquid, solid, or gaseous, become coloured with the most brilliant hues as soon as they are reduced to plates of extreme thinness. In the soap-bubble it is the oleaginous particles floating on the surface which thus become coloured, but Newton showed that thin plates of air were similarly capable of showing colour, and that the thinner the plates were the more brilliant were the tints. We may see this in the soap-bubble, which becomes more beautiful as it gets larger and thinner. By placing a convex lens of large size on a flat plate of glass, Newton observed that rings of different colours were formed round the spot where the two pieces of glass touched.

Fig. 12.—Newton’s Rings.

By measuring the convexity of the lens and the diameter of the various rings, Newton was enabled to tell to a minute fraction the exact thickness of the plate of air corresponding to the different colours. The glasses being placed in position, a ray of a particular colour—red, for instance—was thrown upon the surface. The result was a black spot at the point where the two surfaces touched, and surrounding it at various distances were several rings alternately red and black. Calculating the thickness of the plates of air at the part where the dark rings made their appearance, Newton found that their dimensions were in the proportion of the even numbers two, four, six, eight, &c.; while the red rings showed figures corresponding to the odd numbers. Although trammelled by the corpuscular theory, Newton’s deductions from these experiments show that they can only be accounted for by the undulatory hypothesis. Thus the thickness of the plate of air at the first red ring is that of the red wave, the thickness at the second that of two red waves, and so on; so that in order to arrive at the thickness of the red wave we need only measure the distance between the portions of the glasses where the first red ring occurs.

This experiment, was applied to the measurement of all the waves. Whenever they were reflected on the glasses a parallel series of rings was formed, but it was found that the first ring was more or less distant from the central spot, according to the colour used. The red ring was the largest; the orange, yellow, green, blue, indigo, and violet, following in the same sequence as in the spectrum. The word “thickness” seems hardly fit to apply to dimensions arrived at by Newton in his experiments, so infinitely small do they appear to be, yet their correctness has never been impugned, although the experiments have been repeated by the philosophers of all countries. The waves of red light are so small that 40,000 of them go to an inch, and those of violet light situated at the other end of the spectrum are still smaller, measuring only the 60,000th part of an inch.

The waves of the other colours are between these two, while the wave of white light, which is a mixture of them all, is just half-way between the two.

Thus was the physical cause of the various hues of colour discovered by this great man, revealing as it does the singular and mysterious analogy between sound and light. The rays of light, like the waves of sound, produce a different effect, according to their length, by causing quicker or slower pulsations in the nerves of sight, just as musical sounds vibrate upon the drum of the ear with different velocities.

This is not all, for the relationship between sound and light does not cease here: we have as yet only spoken of the size of the undulations, and have only shown how their dimensions are connected with the sensation of colour; but there are other things to be considered, for on investigation we find that not only do the different coloured waves vary in the length of their undulations, but also in the number that take place in a given time.

The perception of sound is produced by the action of the drum of the ear, which vibrates sympathetically with the pulsations of the air that have been originated by the vibrations of the sounding body; and the perception of light is produced in a similar manner by the vibrations originating in a luminous body, and propagating themselves through the luminous ether until they reach the nerves of sight. The number of these pulsations taking place in the eye has been accurately determined in the following manner. Let us suppose that we are looking at a coloured object—let us say, a red railway signal-lamp; from the lamp to our eye there flows a continuous line of luminous undulations; these undulations enter the eye and become depicted on the retina. For every wave that passes through the pupil, there is a separate and corresponding vibration of the optic nerve, and the number of these vibrations that take place in the course of a second can be easily calculated if we know the velocity of light and the breadth of the waves. We have before found that light travels at the rate of 185,000 miles per second; it therefore follows, that a series of undulations 185,000 miles long pass through the pupil every second; consequently the number of vibrations per second is arrived at by calculating how many waves measuring the 40,000th of an inch—that being the length of a wave of red light—are contained in 185,000 miles. The following table, showing the number of waves passing into the eye per second for the different colours, will interest the student:—

Whatever theory we may adopt to explain the phenomena of light, we arrive at conclusions that strike the mind with astonishment and admiration. According to the corpuscular hypothesis, it was supposed that the molecules of light were endowed with the power of attraction and repulsion, that they possessed poles and centres of gravity like the earth, and that they had other physical properties that could only be given to ponderable matter. Starting with these notions, it is difficult to divest oneself of the idea of sensible size, or to induce the mind to conceive particles so extremely small as those of light would necessarily be if the theory of emission were accepted. If a particle of light weighed a grain, it would produce by means of its enormous velocity the effects of a cannon-ball weighing 120 lbs., travelling at the rate of 300 yards per second. How infinitely small would be these particles, seeing that the most delicate optical instruments are submitted to their action for years without being injured!

If we are astonished at the extreme smallness and prodigious rapidity of the luminous molecules whose existence is necessitated by the corpuscular theory, the numerical results of the undulatory hypothesis are not less surprising. The extreme smallness of the distance between the waves, and the inconceivable quickness of their undulations, although both are easily calculated, must raise in the mind of the student feelings of the utmost wonder and admiration.

Colour, then, simply results from the difference in the rate of vibration of the rays, as Professor Tyndall observes in his lectures on the “Analogy between Sight and Sound,” the impression of red being produced by waves that undulate a third less rapidly than those which produce the sensation of violet.

CHAPTER IV.
LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM.

The solar spectrum may be compared to a battle-field with an army drawn up upon it ready for action. In the centre we find the luminous rays, on one side the light troops which produce chemical effect, and on the other the heating rays, which may be compared to squadrons of heavy cavalry. Close by the light brigade are the magnetic rays, which are a corps of skirmishers, sometimes appearing, and at others hiding themselves from view in a very mysterious manner.

But to drop metaphor, we shall find on examination of the spectrum that the three great forces—heat, light, and chemical effect—are regularly distributed over three different portions of this wonderful band of colour.

Before Fraunhofer the intensity of the light of different parts of the spectrum remained undetermined with any degree of accuracy; but this philosopher, by the use of a very delicate photometer, obtained the results given below.

The maximum of luminous effect is situated just at the junction of the yellow and orange. Taking this spot as its starting-point, it gradually decreases on each side until it ceases altogether at the extreme red and violet.

With respect to the calorific portion of the spectrum it was for a long time supposed that the heat-giving properties of any part were in direct proportion to the amount of its luminous effect; but Sir John Herschel proved by a long series of experiments that the heat of the spectrum gradually increased from the extreme violet to the extreme red, and that passing this point it still further increased until it attained its maximum at a point where not a single ray of light existed. From these grand experiments he adduced the important conclusion, that in solar light there existed invisible rays, which produced heat, and which possessed even a less degree of refrangibility than the extreme red rays. Sir John Herschel then tried, but unsuccessfully, to determine the exact refrangibility of the invisible heat rays.