L «BRAnY OF THE WONDERS OE OETICS. 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. PAKT 1 THE PHENOMENA OF VISION. CHAPTER I. PA«B THE EYE » • • • ••»••• 15 CHAPTER II. THE STRUCTURE OF THE EYE . . 22 CHAPTER III. THE ERRORS OP THE EYE 30 CHAPTER IV. OPTICAL ILLUSIONS • . • . 36 CHAPTER V. THE APPRECIATION OF COLOUR 44 ix X CONTENTS. CHAPTER VI. ILLUSION'S CAUSED BY LIGHT ITSELF CHAPTER VII. THE INFLUENCE OF THE IMAGINATION PART II. THE LAWS OF LIGHT. CHAPTER L WHAT IS LIGHT? •••••••••• CHAPTER II. THE SOLAR SPECTRUM • o • . CHAPTER III. OTHER CAUSES OF COLOUR . CHAPTER IV. LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM • CHAPTER V. THE LAWS OF REFLECTION.- — MIRRORS CONTENTS. CHAPTER VI. METALLIC BURNING MIRRORS • • CHAPTER VII. LENSES • • •••••• CHAPTER VIII. OPTICAL INSTRUMENTS. THE SIMPLE AND COMPOUND MICRO- SCOPE. THE SOLAR AND PHOTO-ELECTRIC MICROSCOPE . CHAPTER IX. THE TELESCOPES OF GALILEO, GREGORY, NEWTON, HERSCHEL, LORD ROSSE, AND FOUCAULT PART III. NATURAL MAGIC. CHAPTER L THE MAGIC LANTERN CHAPTER II. THE PHANTASMAGORIA xii CONTENTS. CHAPTER III. OTHER OPTICAL ILLUSIONS 196 CHAPTER IV. THE PROPERTIES OP MIRRORS .. o ....... . 216 CHAPTER V CHINESE SHADOWS 223 CHAPTER VI. POLYORAMA — DISSOLVING VIEWS — DIORAMA ...... 231 CHAPTER VII. THE STEREOSCOPE 236 CHAPTER VIII. THE CAMERA OBSCURA AND CAMERA LUCIDA ...... 242 CHAPTER IX. THE SPECTROSCOPE • . . 249 CHAPTER X. SPECTRES — THE GHOST ILLUSION . . • 264 LIST OF ILLUSTRATIONS. PIG. PAQB 1. Section of the Eye 24 2. A Camera Obscura 27 3. The Phenakistiscope 54 4. Disc of the Phenakistiscope 55 5. Solar Spectrum Frontispiece 6. Absorption of Light by Sodium Vapour . . ib. 7. Action of a Prism on the Solar Kays . . . ib. 8. The Recomposition of Light 86 9. Eecomposition of Light by means of a Concave Mirror 87 10. Recomposition of Light by means of a number of Mirrors 88 11. Newton's Disc 89 12. Newton's Rings 95 13 Reflection from Plane Surfaces 107 14. Refraction ; 108 15. Experimental Proof of Refraction ib. 16. The Effects of Plane Mirrors 109 17. Reflection from the Surface of Water 110 18. Concave Mirror Ill 19. Conjugate Foci 113 20. Virtual Focus 114 21. Concave Mirror ib. 22. Magnifying Effect of Concave Mirrors 115 23. The Reversal of real Images ib. 24. Diminishing Power of Convex Mirrors 116 25. Burning Mirror 124 26. Double Convex Lens 127 27. Forms of Lenses 128 28. Path of a Ray through a Convex Lens 129 29. Path of Divergent Rays through a Convex Lens . . ib. 30. Conjugate Foci 130 31. Images forme 1 by Convex Lenses 131 32. Magnifying Property of Convex Lenses 132 33. Diminishing Effect of Concave Lenses ib. xiii Xiv LIST OF ILLUSTRATIONS. 34. Cannon of the Palais Royal . . 134 35. FresnePs Lighthouse Apparatus ,136 36. Lantern of a First-Class Lighthouse 140 37. The Compound Microscope 143 38. The Theory of the Compound Microscope .... 144 39. Photo-Electric Microscope 147 40. Solar Microscope . 148 41. The Galilean Telescope 155 42. The Astronomical Telescope 156 43. Section of an Astronomical Telescope 157 44. Section of the Gregorian Telescope 160 45. Gregorian Telescope 161 46. Section oi a Newtonian Telescope ....... 162 47. Herschellian Telescope 164 48. Foucault's Large Telescope 163 49. Foucault's Small Telescope 171 50. Section of the Magic Lantern 179 51. Magic Lantern 182 52. The Phantasmagoria 184 53. The Phantascope 185 54. Phantasmagoria (Robertson) 194 55. Wizard Dance . . 198 56. Nostradamus and Marie de Medicis . 201 57. The Arrangement of the Reversing Prism .... 233 58. The Goat Trick 205 59. How to see through a Brick 207 60. The Polemoscope 210 61. Protection against ill-natured People 213 62 . 218 63. Anamorphosis 220 64. Effect of Cat Paper-work 225 65. Seditious Toys 229 66. Diorama 234 67 237 68. Stereoscope . 238 69. The Principle of the Refracting Stereoscope .... 239 70. The Camera Obscura . # 243 71. Section of Camera Lucida 247 72. The Spectre— an Optical Illusion 269 73. How to produce Spectres 271 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 ex- terior 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 extend- ing 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 15 16 THE WONDERS OF OPTICS. every object that comes within its ken; not only does it reveal to us our own position and that of our sur- roundings ; but, thanks to the discoveries of modern science, it is able to admire, on the one hand, a worlcl of infinite minuteness that remained unknown to us for centuries, and, on the other, the immeasurable immen- sity 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 consi- der 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 mi id 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 re- flected. When the mind is undisturbed, says Buffon, all the parts of the face are in a state of repose ; their pro- portion, unity, and general appearance indicate the pleasing harmony of our thoughts and the perfect calm- ness 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. THE EYE. 17 It is more especially in the eyes, adds this great naturalist, that these signs are manifested and recog- nised. 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 be- come 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 sqaint, 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 tilings 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 iillow 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 B 18 THE WONDERS OF OPTICS. 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 di- rected 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 percep- tible 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 THE EYE. 19 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 har- mony 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 co- lour 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 inves- tigations have failed to bear out the ancient philoso- pher'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 eye- ball 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 plea- sure upon the same object : but in most animals this parallelism is wanting. In some cases the eyes of ani- mals are set almost back to back, rendering it impossi- ble 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 20 THE WONDERS OF OPTICS. strike us more than any other portion of the counte- nance. 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 eye- lid. 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 ex- pression 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 con- stitutes the true beauty of the eye : every one knows THE EYE. 21 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 con- tained 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 fa§ade. By the way, as we are about to describe many illusory wonders, clo 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 say- ing 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. \ 22 THE WONDERS OF OPTICS. 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 plu- mage 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. THE STRUCTURE OF THE EYE. 23 A strong sentiment of pity should, therefore, ani- mate 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 under, standing, 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 pro- vided ! 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 perspira- tion from entering and irritating the organ. The eye- lids 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 thje entrance of dust or any other foreign body with which the eyes might be in- jured. 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 per- fectly 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 THE WONDERS OF OPTICS. 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 oc- cupying 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. 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 ex- I Fig. 1.— Section of the Eye. pansion 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 li^ht 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 0, a bi-convex lens to transmit the rays of light to the retina. It isgener* ally called the crystalline lens. THE STRUCTURE OF THE EYE. 25 From the crystalline lens to the back of the eyeball, ia 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 oppo- site the pupil and lens, is the most delicate and im- portant 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 pro- vided 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 mil- lions of the laminae above referred to. With such won- ders is the world of nature replete, — wonders that we daily and hourly pass by without examination. 26 THE WONDERS OF OPTICS. It is by means of this ingenious and inimitable struc- ture of the eye that external objects pass from the do- main of the material world into that of the mind, and become accessible to every faculty of our brain. Of its own accord, arid 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 distin- guishing instantaneously between the distance of the remotest nebulae 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 natur.e, and con- veys 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 ob- jects 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 oi the eye, while the trees, the roof of the house, and the chim- ney 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 repre- sented 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 THE STRUCTURE OF THE EYE. 27 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 edu- cation of the eye that we see objects unreversed. Others, like the great physiologist Miiller, imagine that as we see everything upside down, and not a single ob- ject only, we have no points of comparison, and practi- cally 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 28 THE WONDERS OF OPTICS. 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 posi- tion of objects around us, without taking into considera- tion the reversed picture of them that is depicted on our retina. The student who takes an interest in the struc- ture 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 look- ing 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 cor- rected 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- THE STRUCTURE OF THE Em 29 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 curvature** 30 THE WONDERS OF OPTICS. CHAPTER 111. THE ERRORS OF THE EYE. It is with our own organization that we shall com- mence 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 re- ceived a sound box on the ear from an opponent of his system of philosophy. If any of our readers were to became 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 end- less 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 beauti- ful or interesting these deceptions may appear, we shall speedily be able to convince ourselves that they are THE ERRORS OF THE EYE. 31 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 turn- ing towards th^j light. From the time that children begin to distinguish ob- jects, 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 obj-ects. The next cause of error that is likely to mislead them is the fact of their seeing every- thing double, a separate image of everything being formed on each eye ; and it can only be by the experi- ence gained through the sense of touch that they can acquire the knowledge necessary to rectify these er- rors, and see those objects single which appear to them double. This error of sight, as well as the first one, is s*-t 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, state of things brought about entirely through ano- ther sense exercising its power over our judgment ; and it is hardly too much to say that, if that sense were de- prived 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 32 THE WONDERS OF OPTICS. 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 ex- periment is easily tried, and is very convincing. Thus we see that an image is formed on both eyes, and w T e consequently see the object, whatever it may be, re- peated 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 con- tinually 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 ex- pression 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 THE ERRORS OF THE EYE. 33 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 al- lowed 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 £he 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 con- tinually forgot his visual impressions, and mistook one thing for another. He was more than two months be- 34 THE WONDERS OF OPTICS. fore 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 repre- sentations of solid objects, he was often extremely sur- prised 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 won- derful a manner, that numberless bets have been made, lost and won, over them. When feeling such repre- sentations of solid objects, the boy would often ask those around him w T hich 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 ah o imagined that there was nothing beyond the room he was in, and could not be brought to comprehend how 7 the house could be larger. When the sight of the second eye was restored to him a year afterwards, he at first saw ever} object of an enor- mous 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 opera- tion, 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 THE ERRORS OF THE EYE. 35 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 im- pressions by the aid of the sense of touch. 86 THE WONDERS OF OPTICS. CHAPTER IV. OPTICAL ILLUSIONS. Besides the errors of sight already spoken of, there are other illusions, which are either common to all per- sons or confined to certain individuals, the knowledge of which will serve as a fitting prelude to the descrip- tions of those which are artificial. The following defecf, for instance, is one which is li tie 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 in- capable of experiencing the effects of the rays of light when they impinge upon it. For objects situated oppo- site 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 par- allel 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' dis- tance 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 OPTICAL ILLUSIONS. 37 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 menti ned, 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 seeiug 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 38 THE WONDERS OF OPTICS. attention is attracted by a particular portion of a land- scape, we see only that, and nothing else. If it is fixed on some subject that we are contemplating in- wardly, 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 inani- mate, 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 pur- poses. The phenomena of ovular 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 impres- sion 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 OPTICAL ILLUSIONS. 39 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 in- stance, in scarlet paper, place it upon a white back- ground and look at it fixelly 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 ex- plained, 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 w T e come to speak of the prismatic spectrum), mixed in certain pro- portions, form white light ; consequently the comple- mentary of orange, which is composed of red and yel- low, will be blue ; of green, which is yellow and blue, red ; of purple, which is blue and, red, yellow, and vice versa. 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 col- oured. 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 40 THE WONDERS OF OPTICS. 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 hprse, was trotting alorjg 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 be* hind a wood. An instant after the observer was terri- fied at seeing a white cavalier on a black horse project- ed on a white cloud at which he was accidentally looking. It may be readily imagined that such an oc- currence, followed up by a succession of unusual events, — such as illness, death, or any other series of misfor- tunes, — might even in the present day add a chapter to the history of the marvellous. OPTICAL ILLUSIONS. 41 To the illusions to which, like the preceding, we are all subject, may be added those resulting from some ab- normal conformation, or some disease of the eye, in those who labour under them. An example of this oc- curs in the case of double or triple vision, many re- markable instances of which are mentioned by Muller, 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, how- ever, 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 experi- ments. When he looked at a clear mark on a whice ground, and gradually walked away from it, not only did the image become indistinct, but it seemed to u:.fold itself into several, independently of many others much 42 THE WONDERS OF OPTICS. 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 dif- ferent 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 pheno- menon 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, OPTICAL ILLUSIONS. 43 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 hdve a supernatural origin. Bartholin mentions the case of a hysterical woman who was afflicted with hemiopia horizontally, and saw all natural objects cut in tw T o, 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 vihen the eye, or the neighbouring parts, are struck or stimulated by friction or electricity. These appearances are experi- enced even by those who have lost their sight. Muiler states that a case was submitted to a legal tribunal to decide whether the luminous sensations which are per- ceptible when we rub our eyes are really light. The matter in dispute was w T hether 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, Hum- boldt 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 cru- elly 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 si- milar 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. 44 THE WCWDEiiS OF OPTICS. 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 absur- dity 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 yel- low 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 call- ing 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 es- sentially 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 sub- THE APPRECIATION OF COLOUR. 45 stance with which we are acquainted. The old saying, that " all cats are black in the dark/' is really a pro- found 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 ne- gro 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 yel- low, and it is by tfoe 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 re- mainder 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 inte- grity. 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 proba- bility 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 insensi- bility 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 i 46 THE WONDERS OF OPTICS. 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 par- ticular 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 hirn 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 differ- ence 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 Hud- dart 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 daugh- ter to a worthy young man of his acquaintance, and that THE APPRECIATION OF COLOUR. 47 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 re- monstrance, 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. Se- veral 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 Si- berian 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 Maga- sin 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 48 THE WONDERS OF OPTICS. 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, Daiton pointed to the trees around them, and declared he could distin- guish 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 Dal- ton's time over five hundred distinctly marked instances of this imperfection have been noticed, and Professor Prevost, of Geneva, has named it Daltonism, an ex- tremely unphilosophical piece of pathological nomen- clature, which has unfortunately received the sanction of too many great physiologists to be abolished. Blind- ness might just as well be called Homerism or Mil- tonisrn. Colour-blindness is much more frequent than is gene- rally 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 in- stance 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 sig- nal. 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 cer- tain 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 Ber- lin who were examined by Leebech, he found five who were quite confused in their notions of colour, and THE APPRECIATION OF COLOUR. 49 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 in- stances 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-blindaess, — one where only two colours are seen, the other where more than two are perceptible. Dau- beny Turberville, an oculist of Salisbury, mentions a case of the former, in which a young girl, like the Mary- port shoemaker mentioned by Brewster, could only dis- tinguish between black and white, everything between the two being of different shades of grey. This girl, sin- gularly enough, could see to read in twilight a quarter af 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 Trough- ton's family were equally incapable of distinguishing any colours but blue and yellow. The cases of colour-blindness where more than two colours are dis'inguishable, 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,, 50 THE WOJnDERS OF OPTICS. 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 inca- pable of perceiving blue and its several hues, and called their defect by a high sounding Greek name, akyano- blepsy, or blue-blindness. Peclet 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 scar- let of the trousers of the French infantry with the leaves of the trees. Yellow appeared to them more bril- liant than any other colour. Doctor Sommer and his brother could not distinguish between red and its deri- vatives and other colours; they could only distinguish between yellow, blue, white, and black. Doctor Ni- choll 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 w r as 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 be- cause in daylight he confounded all light colours ; yel- low 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 THE APPRECIATION OF COLOUR. 51 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 ob* served by Wartmann, a distinguished German oculist, merits our attention. The afflicted person, whom Wart- mann 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 exemptfrom it. Likeso many others who are colour- blind, he could riot 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 fi different action on his retina, Wartmann tried him first with the prismatic spectrum, but he could only distin- guish 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 ap- plied to colour, for we have no possible means of know- ing what colour it is that is really seen by the patient. 52 THE WONDERS OF OPTICS. Thus, for instance, Dr. Whe well 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. ILLUSIONS CAUSED BY LIGHT ITSELF. 53 CHAPTER VI ILLUSIONS CAUSED BY LIGHT ITSELF. When playing about the Christmas fire, children frequently amuse themselves by whirling round and round apiece 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 im- pression made by it at one part of its journey remain- ing 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 re- volving 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. 54 THE WONDERS OF OPTICS. The phenakistiscope may be described (figs. 3 and 4) as consisting of an iron pin a b turning easily on its «vxis, and passing through two holes in a brass rod t g, bent twice at right angles. Attached r * to one end of 'the pin is a disc of card- l^^^Jl£ a board, divided into several equal sec- \j tors, and pierced near its circumference with as many similar sized rectangular holes (fig. 4 ) In each sector the same scene is represented, with this differ- ence, that the movements of the objects e 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 ap- paratus is held before a looking-glass by the handle m. If the disc be now F IG .3._The Phena- rotated by the button b, and the eye kistiscope. 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 work- ing 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 propor- tion 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 ILLUSIONS CAUSED BY LIGHT ITSELF. 55 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 FlG. 4.— Disc of th^ Phenak*stiscope. 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. The second instrument, the thaumatrope, is construct- ed 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 turne 1 on their axes 56 THE WONDERS OF OPTICS. 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 sud- denly 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 de- signs applicable to this little instrument are endless. The third of these instruments, the phantascope, is constructed in accordance with the peculiar power pos- sessed by the eyes of adapting themselves to the dis- tance of the objects they are looking at. Everybody must have noticed that in order to see objects plainly that are placed at different distances w T e insensibly alter the position and focus of the eyes, and that, conse- quently, 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 posi- tion 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 Fee it only as a con- fused 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 inter- posing the finger between the eyes and any object. When we look at the finger, the distant object will seuld not go on if the memory introduced amongst them bril- liant representations of the past in the midst of ordinary domestic scones or the objects familiar to us. We may accou it 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 imper- ceptible. The exercise of the mind in these instances is, THE INFLUENCE OF THE IMAGINATION. 71 however, so rapid that the alternate appearance and disappearance of the two different impressions is com- pletely 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, w T hen 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 seethe 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 72 THE WONDERS OF OPTICS. 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 involun- tary 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 con- necting them together. WHAT IS LIGHT? 73 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 consti- tutes light itself. Any formal definition would rather puzzle than help the student ; we must therefore con- tent 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 per- formed, had accustomed himself for a long time to ima- gine 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. 71 THE WONDERS OF OPTICS. 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 end- less 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 beau- ties 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, disposi- tion, 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, spr.ead like a veil over us, and throw a shadow upon our life. The return of the bright spring sun, the re- appearance 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 pro- gress of mankind from those ancient people who trem- bled at the approach of darkness, and who fervently saluted the dawn with prayers and praises, dow T n to the philosophers of the present age, w 7 ho investigate its effects with so much reverential joy. But wc must cease paying any more attention to the superficial ac- tion 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. WHAT IS LIGHT? 75 It was believed for a long time that light was a com- pact 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 illu- minated by them were supposed to throw them off as if they were endowed with elasticity. Under this hy- pothesis, 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 curi- ous 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 begin- ning of the present century, Fresnel showed, by the most brilliant discoveries the superiority of this theory, and shortly after Arago confirmed him in his demon- strations. According to the undulatory hypothesis, light is not a mass of molecules emitted by a luminous bo ly, 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 produce 1 round the point from whence the sound pro- ceeds. A series of waves are formed which spread not only h>riz mtally, as on the surface of the water dis- turbed by the stone, but in every direction. In fact, in the case of sounds, the waves are so many gradually 76 THE WONDERS OF OPTICS. increasing spheres. In the case of light, when a lumi- nous 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 si- lence, is absolute rest. Many people still believe that light is propagated in- stantaneously, and cannot bring themselves to imagine th^t 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 cer- tain amount of elemen ary 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 ena- bled to perceive them left their surface, and which only reaches us after travelling through space a certain num- ber 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 r of the subject w T as not, like so many of the questions relating to physics, a mere opinion without proof; for Aristotle, in mention- ing 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 philoso- WHAT IS LIGHT ? 77 phers 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 ob- server saw the second light disappear the instant he had extinguished his own, Galileo concluded that light was propagated instantaneously through a distance dou- ble 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 ab- surd, 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 undu- latory hypothesis, believed that light was transmitted instantaneously throughout any distance, and endea- vours 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 inge- nious train of reasoning proves that whether the trans- mission 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. 78 THE WONDEKS OF OPTICS, The frequent occupations of the first satellite of Ju- piter, the discovery of which was almost consequent upon that of lenses, furnished Romer 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, wlion 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 pow r er 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 iEsop, 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 co fidence that u 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 tho se 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, Romer, a celebrated Danish astronomer, was engaged in a se- ries of observations, the object of which w r as to deter- mine the time which one of these bodies took to revolve round its planet. The method employed by Romer was to observe the successive oc ultations of the satellite, WHAT IS LIGHT? 79 and to notice the interval that elapsed between each of them. But it at last happened that the interval be- tween the two occupations, 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 Tvas receding from the planet, while it became propor- tionally cut short during the rest of the year. Romer 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 in- stant when it really took place, but that it sometimes appeared to happen later — that is to say, after an in- terval of time sufficiently long to allow the light that had left the satellite immediately after its disappear- ance, to reach the eye of the observer. Hence it be- came evident that the farther oft' 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 mcment of the disappearance of the satellite is that of the commence- ment of the occultation, and that the ircment of the arrival of the last portions of light is tl at when the commencement of the occultation is observed. It was thus that Rom er explained the difference be- tween the calculated and observed time of the occulta- tion, and he saw that he was on the threshold of a great discovery. In a word, he saw that light propa- gated itself through space with a certain velocity, and that the fact we have just mentioned furnished the pre- cise 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. 80 THE WONDERS OF OPTICS. It must be remembered when considering this sub- ject, 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 trans- mission 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 float- ing body, which we shall find is but little, if at all, influ- enced 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 in- stance 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 distin- WHAT IS LIGHT? 81 guish at an early period the exact difference between true movement and mere undulation ; and we must re- member that although the waves of light are propa- gated 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 cor- puscular 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 after- wards 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 p. aceably, Marat allowed himself to be carried away by his temper, which was naturally very violent. Every argument ad- vanced 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 pre- vent 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 F 82 THE WONDERS OF OPTICS. 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, a^d 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 wilV 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 WHAT IS LIGHT? 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 demoLStrate the truth of the undulatory theory, by showing that it could not be explained by any other. S4 THE WONDERS OF OPTrCS. CHAPTER 11. 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 p 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 THE S"LAR SPECTRUM. 85 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 proceed- ing 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 in- dicated 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 rain- bow, 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. Bjfore 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 86 THE WONDERS OF OPTICS. 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 com- pletely 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 Recom position of Light. Another experiment in the same direction consists in THE SOLAR SPECTRUM. 87 reuniting the colours by causing them to pais 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 ex- periment of burning a piece of paper with an ordinary magnifying-glass. Instead of using a lens, you can, if you please, em- ploy a concave mirror, using the ground glass or card- Fig. 9. — Recomposition ot Light by means of a Concave Mirror. board screen, as before. The colours reflected by the mirror unite at its focus, and produce a brilliant w r hite spot in just as conclusive a manner as in the other ex- periment. A fourth experiment, which is somewhat more diffi- cult 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 direc- tion. By directing each of the seven rays, one by one, upon the same point, you may observe the gradual de- 88 THE WONDERS OF OPTICS. composition of the coloured light. The effect obtained by adding the last colour to the mixture is quite magi- cal, 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 compo- nents 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 dis- appear, and a greyish hue will be seen, which will ap- proach 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. From these experiments it follows that the colours THE SOLAR SPECTRUM. 89 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 Fig. 11. — Newton's Disc. plants, for instance, must not be regarded as being really green in themselves, but as being capable of ab- sorbing 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 pos- sesses the property of absorbing red light, and reflect- ing green. A green leaf placed in red light becomes al- most 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 90 THE WONDERS OF OPTICS. 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 trans- formed 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 es- sential property of bodies. We have already spoken of complementary colours, or those which it is necessary to add together in order to produce w T hite light. Blue, for instance, is complemen- tary to orange, red to green, violet tp 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 THE SOLAR SPECTRUM. 91 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 effec.t 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 day- light 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 v oom lighted entirely with one or two lamps of this kind will form a laboratory for some very singular ex- periments. 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 92 THE WONDERS OF OPTICS. being first lighted with ordinary daylight, the lamps above mentioned should be brought in, and the day- light carefully excluded, when an astonishing metamor- phosis 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, i very 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 lumi- nous figures, such as those of flowers or animals, they THE SOLAR SPECTRUMc 93 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 wav 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. 94 THE WONDERS OF OPTICS. 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 hear- ing. On the one hand, the differences in the lengths of the sonorous waves constitute the variety of note per- ceptible 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 experi- ments 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 dis- appear in a breath, hardly seem worthy of the attention of a thinker, and still less the examination of a philoso- pher ; but it is nevertheless true that Newton made ex- periments 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. OTHER CAUSES OF COLOUR. 95 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 thin- ness. 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 beau- tiful as it gets larger and thinner. By placing a con- vex 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 di- ameter 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 sur- faces touched, and surrounding it at various distances were several rings alternately red and black. Calcu- lating 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 num- bers. Although trammelled by the corpuscular theory, Newton's deductions from these experiments show that 96 THE WONDERS OF OPTICS. they can only be accounted for by the undulatory hypo- thesis. 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 or- der to arrive at the thickness of the red wave we need only measure the distance between the portions of tK 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 ex- periments, 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, pro- duce 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 OTHEK CAUSES OF COLOUR. 9T 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 dif- ferent coloured waves vary in the length of their undula- tions, 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 per- ception of light is produced in a similar manner by the vibrations originating in a luminous body, and propa- gating 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 cal- culated 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 cal- culating 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 :— 6 98 THE WONDERS OF OPTICS. Extreme red Red . Orange Yellow Green Blue Indigo Violet 458,000,000,000,000 waves per second. 477,000,000,000,000 506,000,000,000,000 535,000,000,000,000 " 577,000,000,000,000 " 622,000,000,000,000 « 658,000,000,000,000 " 699,000,000,000,000 " Extreme violet . 727,000,000,000,000 Whatever theory we may adopt to explain the phe- nomena of light, we arrive at conclusions that strike the mind with astonishment and admiration. Accord- ing 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 ve- locity 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. OTHER CAUSES OF COLOUR. 99 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. 100 THE WONDERS OF OPTICS- CHAPTER IV. LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PRO- PERTIES 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 squad- rons of heavy cavalry. Close by the light brigade are the magnetic rays, w;hich 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 dif- ferent parts of the spectrum remained undetermined with any degree of accuracy ; but this philosopher, by the use of a very delicate photometer, obtained the re- sults 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 PROPERTIES OF THE SPECTRUM. 101 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 con- clusion, 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 deter- mine the exact refrangibility of the invisible heat rays. Sir Henry Englefield compared these results, and obtained the following figures : — Blue . . . Green Yellow . . Red . . . Beyond the red Berard obtained similar results, but he at first found that the maximum of heat was just at the end of the extreme red, and that beyond it the air was only about one-fifth warmer than the ordinary temperature. Sir John Herschel attributed these discordant results to Berard having used a thermometer with too large a bulb ; he accordingly repeated his experiments with other instruments with long narrow bulbs, and arrived at similar results to those obtained by the English philosopher. We will now pass on to the physical properties of the other end of the spectrum. Towards the end of the last century, Scheele, a Swedish philosopher, remarked that 56 deg. Fahr. 58 62 72 79 " 102 THE WONDERS OF OPTICS. chloride of silver was blackened more quickly by the violet portion of the spectrum than by any other. In 1801, Ritter of Genoa, in repeating certain experiments made by Herschel, found that a much stronger black- ening effect was produced at a point beyond the violet, and that the discoloration was produced with less in- tensity by the violet and still less so by the blue, the change gradually decreasing till the red ray was reached. He also found that when slightly blackened chloride of silver was exposed to the effects of the red rays, or even in the space beyond, its colour was restored to it. From these facts he drew the conclusion that in the solar spectrum there existed two kinds of rays, one at the red extremity, which favoured oxygenation ; the other, at the blue end, which possessed the contrary properties. He also found that when phosphorus was placed in the invisible rays beyond the red, it gave off fumes of oxide, which were immediately extinguished when it was transferred to the other end. On repeating the experiment with chloride of silver, Lubeck found that the tint varied according to the colour in which it was placed. Beyond or in the violet ray it became brownish red, in the blue it became bluish or bluish grey, in the yellow it remained white, or became slightly yellow and reddish in or beyond the red ray. When he used prisms of flint glass, the chlo- ride of silver was discoloured beyond the visible limits of the spectrum. Without being aware of Hitter's experiments, Dr. Wollaston obtained the same results by acting on chloride of silver with violet light. In continuing his researches he discovered that gum guaiacum was also influenced by the chemical rays of light. The magnetic influence supposed to be exerted by the solar rays still remains without positive proof, although numbers of philosophers have experimented in this PROPERTIES OF THE SPECTRUM. 103 direction. More than fifty years ago Dr. Morichini announced that the violet rays of the solar spectrum possessed the property of magnetizing steel needles that were previously free from magnetism. He pro- duced this effect by concentrating the violet rays upon one-half of each needle with a convex lens, taking care to keep the other half concealed beneath a screen. After having continued this experiment for more than an hour, the nee lies were found to be quite magnetic. Dr. Somerville tested Morichini's experiments by covering one-half of an unmagnetized needle an inch long with a piece of paper, and exposing the uncovered half to the violet rays of the spectrum, and found that the needle became magnetic in the course of a couple of hours, the exposed end being the north pole. The indigo rays produced almost the same effect, but the blue and green rays were much less powerful. When the needle was exposed to the yellow, orange, red, and invisible rays beyond the red, no magnetic effect was produced, although the experiment was continued for three days. Pieces of chronometer and watch springs were submitted to the same influences with a similar result ; but when the violet rays were concentrated upon the needles and pieces of spring with a lens, the time necessary for magnetizing them was greatly reduced. Baumgartner of Vienna and Christie of Woolwich also repeated these experiments. The latter philosopher found that when a needle of magnetized steel, copper, or even glass, vibrated by force of torsion in the rays of the sun, the arc of vibration diminished much more quickly than when the experiment was conducted in the shade. The sun's rays appeared to have the greatest effect upon the magnetized needle. From these results Christie concluded that the solar rays were capable of exerting a certain amount of magnetic influence. These experiments were afterwards fully confirmed 104 THE WONDERS OF OPTICS. by those of Barlocci and Zantedeschi. The former found that a natural magnet which was capable of sup- porting a pound weight, had its power almost doubled by exposure to strong sunlight for four-and-twenty hours. Zantedeschi exposed a magnet which would carry fifteen ounces to the sun for three days, and in- creased its power two and a half times. These experi- ments seem almost to decide the fact of the power of white and violet light to induce magnetic force; but a series of researches by a philosopher who without doubt is greater than any of those already mentioned, seems to throw some doubt on the facts we have related above. Before concluding, we must add a few more facts re- lating to the existence of invisible rays at both ends of the spectrum. " The visible portion of the spectrum," says Dr. Tyndall, in one of his Royal Institution lec- tures, " simply marks an interval of radiant action, the rays existing in which bear such a relation to our visual organs, as to be capable of exciting in them the sensa- tion of light. Beyond this interval, in both directions, right and left, the radiant action continues to exercise itself, but the rays emitted are dark, in consequence of their exerting no influence on our eye. Those that ex- ist beyond the red ray are capable of producing heat, while those that are beyond the violet excite chemical action. These invisible violet rays can be actually made perceptible to the eye, or, in other words, the un- dulations or waves proceeding from this end of the spectrum can be made to strike against certain sub- stances and induce luminous vibrations, so as to connect the dark space beyond the violet with a brilliantly illu- minated band. I have here a substance capable of effecting this change. The lower half of this sheet of paper has been moistened with a solution of sulphate of quinine, the other half being left in its ordinary condition. I will now hold the paper in such a manner that the line that separates the prepared half from the PROPERTIES OF THE SPECTRUM. 105 other shall cut the spectrum in two halves horizontally. The upper half will remain unaltered and may be readily compared with the lower half, upon which you will see the spectrum prolonged beyond its ordinary limits. The effect produced is the addition of a splendid band of fluorescent light, which extends over a space of several inches, which but an instant before was a dark "mass. I withdraw the prepared paper, and the light disappears ; I replace it, and the light shines forth once more ; showing us in the most brilliant way that the visible limits of the ordinary spectrum are not the lim- its of radiant action. " I plunge a pencil into the solution of sulphate of quinine, and I pass it over the paper. You see that wherever the solution falls, the light bursts forth. The existence of these rays has been known for a long time. Young was familiar with them, and subjected them to experiment ; but it is to Professor Stokes that we are indebted for a complete series of researches on this sub- ject. It was he who first made those invisible rays visible, as we have done." In the same way the Professor proceeded to show that the heat rays were invisible by passing a beam of sun- light through a solution of iodine in spirits of wine, which, although it completely stopped all light, allowed the heat rays to pass iminterruptedly. By collect- ing these invisible rays into a focus by means of a lens, Dr. Tyndall was enabled to ignite various combustible bodies. Thus we see the reason why certain rays produce certain effects on the eye, each particular degree of re- fraction causing a different set of vibrations, resulting in a different sensation for every part of the spectrum, and reproducing the effect of various colours on the optic nerve. In the following chapters we shall con- clude our account of the different colours in the spec- trum and of the laws of light. 106 THIS WONDERS OF OPTICS. CHAPTER V. THE LAWS OF REFLECTION. MIRRORS. When a ray of light falls obliquely on any polished surface, as that of a mirror, a piece of water, a plate of burnished metal, or any other reflecting substance, the ray, like an elastic ball, is immediately projected in a contrary direction to that in which it fell. Moreover, the direction in which it is reflected is at right angles to the surface, and in the same plane as that of the ray in the first instance. This experiment may be tried very easily, and will show the reason for the two fol- lowing laws. 1. The angle of incidence is equal to the angle of re- flection, and vice versa. 2. Reflection can only take place in one direction — in that of the incident rays, both of which are always in a plane perpendicular to the reflecting surface. The following figure will assist the student in per- forming experiments on the reflection of light from flat surfaces. The ray A B falling obliquely on the horizontal mirror, is reflected upwards at the same angle in the direction b. c. This may be proved geometrically by placing a graduated circle in a vertical position in the plane A B c, when we shall find that the angle A B D formed by A B (the incident ray) with the perpendicular D B is equal to the angle formed by this perpendicular line and the reflecting ray B c. You may also prove THE LAWS OF REFLECTION. — MIRRORS. 107 in the same way that these three lines are all -in the same vertical plane. Fia. 13— Reflection from Plane Surfaces. Let us now examine the effects of light reflected from plane surfaces. We must first, however, notice a certain optical illusion to which we are continually falling a prey, almost without our knowledge. We always fancy objects to be in reality .in the place wh(re we see them, and, in spite of our having already enume- rated a large number of these deceptions, we must still add one more to the list. In reality we rarely see objects in the place where they really are ; for if by the effect ot reflection, refraction, or any other cause, the rays of light are made to deviate from their course, we no longer see the object from which they proceed in its real position, but in the direction taken by the lu- minous pencil at the moment of entering the eye. For instance, if the ray A B is bent during its passage to the eye at b, and consequently reaches it in the 108 THE WONDERS OF OPTICS. direction B c, it is at A 1 , and not at A, that we shall see the object from which it proceeds. Ev^ry ray of light which passes out of a medium of a certain density into Fig. 14. — Refraction another of a different density is bent from its primary course, or, in scientific language, it is refracted. The Fig. 15. — Experimental Proof of Refraction: experiments we made in a former chapter on the properties of the prism are founded on this principle. THE LAWS OF REFLECTION. — MIRRORS. 109 The law may be easily illustrated by allowing a ray of light to fall upon the surface of a. vessel of water, as shown in the preceding figure. The light of the stars and planets undergoes a similar deviation when passing in its course through the earth's atmosphere ; and at the moment we see the rising of the sun, the moon, or a star, they are in reality still Fig. 16.— The Effects of Plane Mirrors. below the horizon. Our eyes consequently are still deceiving us, no matter what part of the domain of op- tics we may enter. THE WONDERS OF OPTICS. There are two kinds of mirrors — plane and curved. We will first examine the properties of the former sort, being those which are ordinarily applied to the usages of every-day life. In the figure in the preceding page we have a young lady looking at her reflection in a tall cheval glass. Every point upon the surface of her clothes and face is reflected back to her eye from the surface of the tin amalgam which has been applied to the back of the mir- ror by the looking-glass maker, for the purpose of ren- dering the image of the object more brilliant than if the glass alone were used. The rays which proceed from every one of these points strike upon the surface of this metallic layer, are stopped by its opacity, and are re- FiG. 17.— Reflection from the Surface of Water. fleeted back to the eye at an angle equal to that at which they strike the surface. The image seen by the eye is formed, consequently, by the reflection of everv o4 of THE LAWS OF REFLECTION. — MIRRORS. Ill these rays ; and as we always see objects in the direc- tion taken by the luminous ray at the moment it enters the eye, we fancy we see objects before us that are really behind, or on each side of us. For instance, the ray starting from the left foot of the young lady in the figure is reflected from the point indicated on the sur- face of the glass, but the eye does not stop here, but sees the foot at an equal distance beyond the mirror. The same thing takes place, not only with glass, but with all substances having polished surfaces. Still water, which to all intents and purposes has a polished sur- face, reflects the objects within its range as perfectly as a mirror. The preceding observations apply to all plane re- flecting surfaces ; but there are other sorts of mirrors, whose effects are of a more interesting nature, and which we must hasten to describe — we allude to those whose surfaces are either convex or concave. Curved mirrors are made of a great variety of shapes, but for the present we shall only describe those which are spherical. Spherical mirrors may of course be either concave or convex. Fig. 18.— Concave Mirror. Suppose the arc M N (fig. 18) to be movable round the point o, this revolution will describe the surface of the mirror. The central point c of the hollow sphere 112 THE WONDERS OF OPTICS. of which the mirror forms part, is called the centre of curvature, the line L the principal axis. By remem- bering these very simple definitions, we shall be able to understand the action of these mirrors without the slightest difficulty. To understand how the rays of light are reflected from the surface of the mirror n m at the point F, which is called the focus, we have only to consider the mirror as consisting of an infinite number of facets, all inclined towards that particular point, and forming by reason of their immense numbers a regular spherical surface. In considering the mirror from this point of view, we can immediately see that, on account of the inclination of the supposed facets, the rays that they receive are all reflected back again at the same point ; and it may be proved geometrically, that when the incident rays are parallel the focus will be situated somewhere on the line c, its position depending on the curvature of the mirror. If, therefore, we receive on a spherical mirror a pencil of sunlight, the rays which compose it may be regarded as parallel, the sun being at so great a distance from the earth ; it follows that these rays will all be reflected together in a particular point, viz., at F, and if any object be placed there it will be illuminated with great brilliancy. The laws governing the reflection of heat being nearly similar to those regulating the action of light, the rays reflected from a burning body will ignite any inflammable substance placed at the point F. The focus for parallel rays is called the principal focus of a mirror. Having described the effects of parallel rays, let us now see what happens when the source of light is close to the mirror. If it is placed at a very small distance, the luminous rays are divergent instead of parallel, and their meeting point becomes changed in accordance with the laws laid down at the beginning of THE LAWS OF REFLECTION. — MIRRORS. 113 this chapter. That is to say, the focus will approach more or less to the centre of curvature c, according as the source of light is placed nearer to or further from the mirror; consequently, in the case of the candle in fig. 19, instead of uniting at F, the rays will meet at /, a point situated somewhat nearer the mirror than the principal focus. If, instead of placing the light at A, we place it at/, we shall find the rays will be concentrated at the point A. Thus the foci are consequently related to each other, and are hence called conjugate foci. It will be readily seen that a spherical mirror may have an infinite number of conjugate foci, according to the dis- tance of the source of light. It is also clear, that if we cause the light to approach the mirror, the focus will also approach it. Fig. 19.— Conjugate Foci. Continuing our experiment, we shall find that when the candle passes the principal focus so 'as to be between it and the mirror, the reflected rays first become parallel and then divergent, and cannot consequently produce any focus beyond the mirror, but are reflected in the way shown in fig. 20. In experimenting on the plane mirror, we imagined we saw the object at a certain distance behind it ; the same thing happens when we see ourselves reflected in 114 THE WONDERS OF OPTICS. a concave mirror, and the particular point at which we suppose we see our reflection is called the virtual focus. Fig. 20.— Virtual Focua. If instead of a candle we place our head before a concave mirror, we shall see ourselves magnified as in fig. 21. Fig. 21. — Concave Mirror, We shall easily see how this happens by tracing the paths of the rays in fig. 22. THE LAWS OF REFLECTION. — MIRRORS. 115 Fig. 23.— The Reversal of real Images. 116 THE WONDERS OF OPTICS. eye in such a way as to appear to proceed from a point beyond the mirror, A. In the same manner the rays re- flected from the chin appear to take their origin from the point B. If, on the other hand, we place ourselves at a distance from the principal focus, we shall produce a reversed and diminished image of our face. This image is not illusory, like the preceding ones, but is real, and may be received upon a screen, as shown in fig. 23. We may easily follow the path of the rays as shown in the figure, and we shall see that the rays forming the images of the church-tower and the terrace below, cross at a certain point. Convex mirrors produce precisely opposite effects, and give a diminished image instead of a magnified one, as may be perceived on examining fig. 24„ Fia. 24.— Diminishing power of Convex Mirrors. METALLIC BURNING MIRRORS. 117 * CHAPTER VI. METALLIC BURNING MIRRORS. The classical student will remember that Archimedes burned the fleet of Marcellus, by means of burning- glasses, from the heights of the fortifications of his native city of Syracuse. Unfortunately, any account of the system of catoptrics, or the science of reflections, employed by the ancient Syracusan in their con- struction is lost to us, and many modern writers have gone so far as to doubt the fact altogether. The knowledge of the properties, however, of concave mirrors which we have just been acquiring, will enable us to form a pretty good guess as to the means adopted by Archimedes for the destruction of the enemy's fleet. The ancients, not having the means of either casting or grinding such enormous mirrors, must have constructed them of a large number of small ones, so arranged that the images of the sun reflected by them would all fall in the same place, or nearly so. In this case, the larger the number of mirrors, the greater would be the burning effect. In order to explain the reflection of rays incident upon the surface of concave mirrors, we supposed them to consist of an immense number of plane mirrors placed in a curve, so that the reflected rays might all meet in one point ; but on examining into the history of burning mirrors, we find that the plan has been adopted in reality in a great number of 118 THE WONDERS OF OPTICS. instances. We have also said, that the reflection of the heating rays was governed by similar ]aws to those influencing the rays' of light ; consequently, by directing a pencil of sunlight upon the surface of a concave mirror, we obtain the maximum of light and heat at the focal point. Many modern writers give the ancients too little credit for their knowledge of optical principles, and late investigations seem to prove that the old school of philosophers were much more learned in these matters than has been generally supposed. The discovery of a rock crystal double convex lens in an Egyptian tomb of great antiquity is an instance of this. Descartes wrote a little treatise to prove that the stories related of the burning mirrors of Archimedes were pure fabrications, although many Latin authors have described them both as being used by that philosopher and in more modern times ; Dion, for instance, who lived in the early part of the sixth century, states that at the siege of Con- stantinople, Proclus burnt the fleet of Vitalian with mirrors of brass ; but the opinion of Descartes seemed to outweigh all other testimony. BufFon, who wished to sift the matter thoroughly, constructed for himself, after many previous experiments on the laws of reflec- tion, a series of mirrors that closely imitated those ascribed to Archimedes. His first memoir, " On the Invention of Mirrors capable of burning at a great Dis- tance/' was published in the Transactions of the French Academy of Sciences for 1747. A few years later he combated both theoretically and practically the opinion of Descartes, in ; memoir containing an account of an immense number of experiments. Before speaking of the extraordinary effects of burning mirrors, it will be as well to do justice to the predecessors of the learned naturalist we have just mentioned, by quoting a passage from the works of Father Kircher, who, 128 years pre- METALLIC BURNING MIRRORS. 119 viously, experimented in this direction with great pa- tience and perseverance, and tried to prove that the stories related of Archimedes were true. " The larger the surface of a mirror," says this philosopher (who, like Huyghens, was a practised astronomer), u the more light it reflects from the objects opposite to it. If it is only a foot square, it will throw a square foot of light upon any wall or screen placed before it. Experiment shows that this light is composed of an infinite number of rays reflected from different points on the surface of the mir- ror. Direct the rays from a second mirror upon the same place as those from the first, and the light and heat will clearly be doubled. They will become trebled if you direct the rays from a third mirror upon the same spot, and so on ad infinitum. In order to prove that the intensity of the light and heat is in direct pro- portion to the number of reflecting surfaces employed, I took five mirrors, and found that on exposing them to the sun I obtained with only one, less heat and light than if I used direct sunlight. With two the light and heat increased considerably ; three gave as much heat as an ordinary fire, and four gave me a still greater effect. I therefore concluded that by multiplying these plane mirrors, I not only obtained greater effects than those got by using parabolic, hyperbolic or elliptical mir- rors, but that I could use them upon objects at a much greater distance. With five mirrors I could obtain these effects at a distance of 100 feet, but what terrible phenomena would have taken place had I used one thousand instead of five?" He ends by begging math- ematicians to experiment in this direction with greater care than they had hitherto done. After Kircher we may cite as an experimentalist with these terrible instruments the French philosopher Vil- lette, who constructed several mirrors, in direct imita- tion of those of Archimedes, for Louis XIV. and other 120 THE WONDERS OF OPTICS. sovereigns. The Journal des Savants for 1679 gives an account of his principal metallic burning mirror in the most eulogistic terms, adding an instance of igno- rance which is singularly quaint and curious. It is of the fourth and most perfect of Villette's mirrors that the Journal des Savants speaks, the first having been bought by Tavernier, and presented to the Shah of Persia, who considered it as one of the rarest and most precious curiosities that he possessed : the second was sold to the King of Denmark, and the third was given by M. Villette to Louis XIV., from whom he received the praises and rewards that were due to his talent and perseverance. " It was thirty-four inches in diameter, and vitrified flints and bricks almost instantaneously, no matter how large they were. It consumed the green- est wood, burning it to ashes in an instant, and fused the most refractory metals with equal ease and quick- ness. Steel, no matter how hard, resisted its power no more than other metals, and melted so quickly that one part burnt away in inconceivably brilliant sparks, some of them forming stars as large as a franc piece, leaving a flowing mass of metal behind. The last made by Villette was still more powerful, being larger and more carefully made. It was forty-four inches in diameter, and three inches and a line deep. Its burning point, or focus, was situated at a distance of three feet seven inches from the surface, and was apparently as large as a five-sou piece ; and it was at this spot, where the rays of light and heat were concentrated into so small a space, that the wonderful effects of its violent power became manifest, the spot of light being of such bril- liancy that the eyes could no more withstand its bright- ness than that of the sun. Besides the property of burning which it possessed in so wonderful a degree, it was capable of exhibiting other effects just as curious as those already related. It had the power of sending METALLIC BURNING MIRRORS 121 the images of objects to a distance of fifteen feet or more, so that a man looking at himself in this mirror with a sti^k or sword in his hand, saw the image of them suspended in the air, apparently ready to strike the ob- server. On seeing such an effect for the first time, the observer could hardly fail to experience the greatest surprise, and even fear; and it is stated that the king having placed himself, sword in hand, before one of these mirrors, in order to observe the effect, was sur- prised to find himself face to face with an armed hand apparently directed against him. When he advanced, the hand seemed to spring forward to meet him. The king could not conceal his sin prise and fright, and after- wards felt so ashamed at being terrified with a mere shadow that he ordered the mirror to be taken away, and could never be prevailed upon to look into it again." The Journal des Savants then goes on quaintly to re- mark on the various startling effects produced by these mirrors, winding up by stating that its powers of reflec- tion were so great, that at night the light of a torch or flambeau was reflected so perfectly that an observer placed at four hundred feet distant could read the small- est print. It also mentions a curious piece of superstition on the authority of a scientific writer of the name of Rob- ertson, who states that it happened at Liege. In read- ing the accounts of these experiments we can see how easily the minds of individuals were affected in those days by the wonderful. It happened while one of Vil- lette's mirrors was at Li6ge, that the latter end of the summer was somewhat rainy, and great fears were en- tertained that a bad harvest and dear bread would be the result. Certain evil-minded people, who had taken a fancy to the mirror and wished to possess it by unfair means, spread the report that the continual rain was entirely caused by its action on the clouds and sun, and 122 THE WONDERS OF OPTICS. that the coming famine must be laid upon the shoulders of its owner and inventor. This absurd idea took such forcible possession of the minds of the populace of Liege, that great mobs collected together, uttering all kinds of maledictions against the mirror and its inven- tor, and at last became so violent that they attacked Villette's house with the intention of smashing his great wcrk, and administering to the unfortunate philosopher the shastisetnent they supposed he deserved. Happily, however, for M. Villette and his mirror, Liege was governed in those days by the Prince Bishop of Co- logne, who was a man of great enlightenment. He put the crowds round M. Villette's to flight by armed force, but he found that the conviction that all the coming mischief would result from the unlucky mirror was so strong, that he was obliged to issue a pastoral pe- remptorily declaring that the idea had originated with a number of malicious people, who spared no pains to propagate it for their own bad purposes, and that it w m a mischievous and dangerous error to ascribe to a mirror a power which only belonged to the Almighty. In 1747, Buffon performed many extraordinary ex- periments with burning mirrors, which were more sur- prising than any that had hitherto been described. They were mostly performed at the Jar din des Planter, at Paris, of which institution Buffon was director ; and many of them are worth describing. On the 3rd of April, at about two o'clock in the af- ternoon, the great mirror was mounted on its stand, and was found to be capable of setting a plank of wood on fire at a distance of 138 feet, when 128 glasses were used, although the light was weak at the time, and the sun was covered with mist. In pursuing these experi- ments great care had to be taken to prevent the by- standers placing themselves within range of its terri- ble power, for several were nearly blinded by looking METALLIC BURNING MIRRORS. 125 at the brilliant focal point of the instrument. The next day, at eleven in the forenoon, although the sun was still covered with mist and fleecy clouds they were able to produce such a heat at 150 feet distant, with 154 glasses, that a pitched plank began to smoulder and would have burnt into flame had not the sun disap- peared at that particular moment. On the fifth of April, at three in the afternoon, with the light much in the same weak condition as it was on the other days, they succeeded in igniting at 150 feet distant, a heap of shavings of deal mixed with charcoal and sulphur, in less than a minute and a half, with 154 glasses. When, however, the sun shone with its natural power, a few seconds were sufficient to effect these results. On the 10th, when the sun was shining pretty power- fully, a pitched pine plank was easily fired with 128 glasses, at 150 feet distant. In this case the ignition was very sudden, and extended over the whole of the radiant spot forming the focus, which at the distance named measured 16 inches in diameter. The same day at half-past two, a pitched elm plank covered in some places with chopped wood, was set fire to with extreme rapidity, and burnt with such violence that it had to be dipped in water before it could be put out. In this experiment 148 glasses were used, at a distance of 150 feet. On the 11th of April, the burning point was fixed at 20 feet distant from the mirror, and combustible sub- stances were easily burnt with only 12 glasses. With 21 glasses a half-burnt elm plank was set fire to, and with 45 a piece of tin weighing six pounds was almost immediately melted. Silver sheet was fused, and an iron plate was made red-hot with 117 glasses. In giving an account of these interesting experiments, Buffon expresses his conviction that at 50 feet it would have been easy to have melted metals if all the glasses 126 THE WONDEKS OF OPTICS. of the mirrors had been used. When used at that dis- tance, the burning spot was six to seven inches in diameter. He also noticed that when metals were melted, part of them were dissipated in brilliant vapour, which was so thick as to cast a shadow on the ground, although it seemed to be as bright as the sun itself. When the sun was at its full strength, and all the glasses were brought into requisition, wood was set on fire at a distance of over 200 feet, and metals and mine- rals were fused at 40 and 50 feet. Hence the possi- bility of making and using these mirrors as Archimedes was said to have done, was proved practically by the great naturalist. Fig. 25 represents a burning mirror in action. Robertson, an English philosopher, residing in France during the days of the first Republic, reconstructed the mirrors described by historians as being used by Archimedes, and the results he obtained were thought sufficiently important by the Council of the Department of Ourthe to merit an attentive examination by two members of their body, who reported in favour of their being used as instruments of war. It would be possible to pursue this subject still fur- ther, and give an account of numerous experiments made on burning mirrors by various philosophers, but we must not forget that it is light and heat that we have more especially to deal with in the present work. Already we have possibly strayed from our path a little too far, but the two influences are so closely connected with each other that it is almost impossible to speak of them separately when reflection is in question. LENSES. 127 CHAPTER VII. LENSES. The word lens is derived from the Latin name of the seed of the Ervum lens, or ordinary lentil. When eat- ing this wholesome vegetable, almost every one has no- ticed that its shape is exactly that of a double convex lens, as represented in the following figure : — Fig. 26. — Double Convex Lens. Perhaps it would be more correct it we were to say that a double convex lens is like a lentil, rather than turn the comparison the other way, seeing that this little seed has given its name not only to the particular- shaped glass depicted above, but also to some five others more or less analogous to it. In fig. 27 we have the different forms of lenses 128 THE WONDERS OF OPTICS. shown in section. The first is the double convex lens, the second the plano-convex, the third and sixth the concavo-convex, the fourth the double concave, and the 14 9 L 6 * Fia. 27.— Forms of Lenses. fifth the plano-concave. A crossed lens is a double convex lens whose one side is more convex than the other. The third lens is also called meniscus. The properties of the first, second, and third are similar; that is to say, they cause parallel rays of light passing through them to converge at a certain point, called their focus ; while the three others have a diver- gent action on rays passing through them. By exa- mining the path of the rays through these lenses, we shall find that the first three magnify objects seen through them, while the latter have the contrary effect. As in the case of the curved mirrors, the rays falling on the surface of a convex lens may be either parallel, divergent, or convergent. In the case of parallel rays, as depicted in the following figure, they are represented as meeting at a point beyond the lens, which is called the sidereal focus, or the focus for parallel rays. It is generally found by causing the image of the sun or of some distant object to be thrown by the lens upon a screen, or by knowing the curvature of the faces, and the refractive power of the glass. Every ray on striking the surface of the lens is re- fracted inwards, until it meets with its companions at LENSES. 129 the focus F, in accordance with the law of refraction, by which a ray of light passing from one transparent medium, such as air, to another which in this instance is glass, becomes refracted or bent in proportion to the Fig. 28. — Path of a Ray through a Convex Lens. relative density of the two mediae. The nearer the ray passes to the edge of the lens, the more it is refracted, the angle of incidence being greater ; the ray through the exact centre being uninfluenced by the form of the glass. Hence they all meet in a single point. Figs. 29 and 30 show the path of the rays when they are divergent and convergent. Fig. 29.— Path of divergent Rays through a Convex Lens. If the rays of light are not parallel, as in the case of the source of light being near the lens, they do not converge so rapidly as when they proceed from a dis- tant object, consequently the focus for near objects is longer in proportion to their distance. In fig. 29 for 130 THE WONDERS OF OPTICS. instance, if a candle be placed as shown, and a screen on the other side of the lens, a point will be found where the image of the candle is seen upon it in a re- versed position. The distance between these two points is always relative, and they are called conjugate foci. Thus, the candle may change places with the screen with a similar effect, as long as the exact position of the two points is preserved. If the candle is placed Fig. 30.— Conjugate Foci. farther off, we mast diminish the distance between the screen and the lens, and vice versa. In fact, the nearer the object, the longer the focus ; the farther it is off, the shorter the focus. Half an hour's experiment with a double convex lens, a piece of white card-board, and a small candle, will teach the student more about the proparties of convex lenses than a chapter of ex- planation. A conmon magnifying glass, or even an old spectacle lens, will serve the purpose of more ex- pensive instruments. We now procaai to speak of the images formed by lenses. In fig. 31 we have a flower placed on one side of a lens. As it is not at an infinite distance, the rays sent out by its various parts are convergent, and not parallel, consequently they do not meet at the sidereal focus, but at a point beyond it, according to the rule already laid down. The rays proceeding from the exact centre of the flower striking the lens exactly in LENSES. 131 the middle at right angles, suffer no change, the others being refracted in proportion to their angles of inci- dence. F'G. 31. — Images formed by Convex Lenses. The rays proceeding from the flower cross each other ut a certain point: hence the image on the screen is reversed. The dimensions of the image will depend on the distance of the object from the lens. This is a fact we meet with every day, when using an opera-glass or a telescope. Images formed by convex lenses upon a screen are called by opticians real images, in contradis- tinction to those which are the result of mere reflection, as in the case of plane mirrors. These latter are known as virtual images and are produced by convex lenses as well as by plain reflecting surface s. In fig. 32, for instance, the unreversed image of the insect seen by the eye is not a real image, but a virtual one, — a fact that might be easily proved by placing a screen in the posi- tion of the eye, when it would be found that no image would be formed. When using an ordinary magnifying-glass we see the virtual image of the object we are looking at, but in the case of a telescope or opera-glass we see the real image of the object, formed by the large lens in front, and 132 THE WONDERS OF OPTICS. reversed again by the arrangement of small lenses next to the eye. Fig. 32.— Magnifying Property of Convex Lenses Double concave lenses produce effects which are just the reverse of those we have been considering. Instead of increasing in thickness from the edges. to the centre, they follow the contrary plan, and increase from the centre to the edges. Consequently, instead of the rays meeting at the focus, they diverge from each other, and gradually spread out, as shown in fig. 33. A B • Fig. 33.— Diminishing Effect of Concave Lenses The above figure shows the path of the rays proceeding from the vase, and meeting the eye at such an angle 134 LENSES. 135 that the virtual image is greatly diminished. Concave lenses, as the student has no doubt already guessed, do not give real images. The effects produced by the action of concave mirrors may be produced with just as much facility by convex lenses. If a body is placed in a focus of a lens which receives the direct rays of the sun, the heat as well as the light will be concentrated at one point ; and if the object is combustible, it will take fire sooner or later, according to the size of the lens. All the experiments mentioned Ly Bulfon as being produced by a concave mirror are equally obtainable with a concave lens. When of sufficient diameter, the most refractory metals, such as platinum or iridium, may be melted and dissi- pated into vapour. Before lucifer matches and vesu- vians were as common as they are now, it was not at all unusual to find smokers carrying a small burning- glass and a piece of tinder, for the purpose .of lighting their pipes or cigars; and there hardly exists a boy who has not lighted a bonfire in the fields or playground by means of an old spectacle lens or telescope glass. Amongst other applications of this property of lenses may be mentioned that of causing guns to fire at a certain time, by arranging a small burning-glass above the touch-hole. In the Gardens of the Palais Royal, at Paris, there is such a gun, so arranged that on sunny days it fires exactly at noon, or, in other words, at the moment the sun comes to the meridian. Every fine day towards twelve o'clock, crowds of Parisians who have nothing to do may be seen bending their steps towards the Palais Royal to set their watches by the gun, w T hich they believe to be superior as a time-keeper to the finest chronometer in the world. There they stand, most of them old fellows with a scar or two about their faces, showing that they have nobly won the rest they appear to enjoy so innocently and calmly with V6H THE WONDERS OF OPTICS. watch in hand, leaning against the railings, and waiting with impatience the moment when true solar noon is indicated by the sharp report of the little piece. Their belief in the correctness of solar time is something astonishing; and if a bystander were to insinuate, rio matter how delicately, that solar time varied slightly every now and then, he would either receive a smile of pitying contempt, or else he would be called out upon the spot. Fig. 34 gives a pretty view of the celebrated cannon of the Palais Royal. We now come to another application of the refract- ing power of lenses, in the way of concentrating rays, which is infinitely more valuable to humanity than Fig. 35.— FresnePs Lighthouse Apparatus. either of those we have just mentioned ; we mean the construction of enormous refracting apparatuses for lighthouse purposes. The first lighthouse of which we LENSES. 137 have any record is that which was erected on the island of Pharos, by Ptolemy Philadelphia, in the year 470 of the foundation of Rome. This was merely a tower, upon the top of which fires were kept burning at night; but as the world progressed, the blazing tar-barrel or wood fire gave place to the carefully-constructed lamp and silvered reflector apparatus, which are fast disappear- ing in their turn before the electric or Drummond light and the refracting apparatus constructed by Fresnel, who was the first to endeavour to abolish the old- fashioned and inefficient metallic mirror from the lanterns of lighthouses. Fig. 35 shows a section of Fresnel's apparatus. A is a plano-convex lens of about a foot in diameter, whose focus corresponds with those of the concentric lenticular rings of glass which sur- round it, and which are seen more plainly in fig. 36. These rings, which are ground and polished with the greatest accuracy, are somewhat in the shape of an ordinary quoit, and are equivalent to a plano-convex lens with the centre portion cut out. This arrangement is so powerful that the distance at which a light pro- vided with it can be seen is only limited by bad weather, the state of the atmosphere and the distance of the horizon. It is common for such lights to be seen at a distance of between fifty and sixty miles. The ap- paratus is mostly arranged in the form of an octagon, and is generally provided with additional reflecting mirrors at those parts above the light which are out of the range of the lenses. The light shining fully in eight directions at one time, can scarcely be missed by any hhip within range; but in order to guard against any possibility of accident, the optical apparatus is often made to revolve by clockwork, so that every point of the ocean is illuminated in turn. By using coloured glasses, or by causing the light to disappear at distinct intervals, different lighthouses may be identified by 138 THE WONDERS OF OPTICS. ships that are out of their reckoning. Fig. 36 repre- sents the interior of the lantern of a first class light- house, showing the arrangement of the lenticular rings round the central lens. If ever the student should pass through Havre, he should not miss the oppor- tunity of seeing this noble apparatus, which is one of the finest ever manufactured* FiG. 36.— Lantern of a First-class Lighthouse. 140 OPTICAL INSTRUMENTS. 141 CHAPTER VIII. OPTICAL INSTRUMENTS. — THE SIMPLE AND COMPOUND MICROSCOPE. THE SOLAR AND PHOTO-ELECTRIC MI- CROSCOPE. The lenses and mirrors whose properties we have been considering in the previous chapters, have been combined in different ways for the purpose of exami- ning objects too small or too distant to be perceived by the human eye. To instruments used for the former purpose the name of microscope has been given, from two Greek words signifying small and to see. In like manner the name of telescope is also derived from two Greek words, meaning distant and to see. Besides these two classes of optical instruments, others have been devised to facilitate the depicting of natural objects, either by means f the pencil or of photo- graphy, or to amuse the e ye by optical illusions. Thus we have the camera obscura, the camera lucida, the magic lantern, the phantasmagoria, and numberless other instruments of the same sort, most of which will be described in the latter part of this book. There are two sorts of microscopes, the simple and the compound ; the one consisting of a single ctmvex lens, and the other of several combinations of both convex and concave lenses. When speaking of convex lenses, we described the properties of the ordinary magnifying glass, or simple 142 THE WONDERS OF Ol'TlCS. microscope. The uses of this instrument are almost too well known to need description. It is used by old people, the lenses of whose eyes have become flattened by old age, by watchmakers for examining the minute portions of their work, by jewellers for the same pur- pose, and by most people for examining maps, engra- vings, and photographs. Simple microscopes are generally mounted in horn, ivory, or metal handles for convenience' sake. Some simple microscopes consist of two or more lenses mounted together in order to in- crease the magnifying power. The student must dis- tinguish between several lenses mounted together in this way, and the true compound microscope, which is a comparatively complicated optical arrangement, as we shall see presently. When two single lenses are thus mounted together, the power of the combination is equal to the powers of each added together. There is good reason for supposing that the simple microscope is a comparatively ancient invention. Sen- eca, who lived in the first century, declares that in his time it was well known that, when writing was looked at through a globe full of water, it appeared larger and blacker. In the eighth century we find the use of mag- nifying spectacles for old people common in most coun- tries, and yet it was only at the beginning of the seven- teenth century that a true optical instrument, in the form of a telescope, was invented. It only needed the placing of two magnifying glasses in a line to discover the principle of the telescope, but nearly a thousand years elapsed after the first introduction of these glasses before an accident rendered the principle evident. In fiV 37 we glass there is of course plenty of space for the body of the man belonging to the magical head. The exhibitor naturally takes especial care never to pass in front of the table, otherwise the lower part of his body would be reflected in mirrors. The polemoscope (from two Greek words signifying " war" and "to see") is another instance of double reflection. It was said to have been invented by Hel- 810 OTHER OPTICAL ILLUSIONS. 211 vetius, about 1637. Fig. 60 will .show the principle of this instrument. The luminous rays coming from a distant object are received upon an inclined mirror, which is elevated above the parapet of a fortification, and are reflected downwards to a second, which is placed at a correspond- ing angle. If necessary, lenses can be interposed, so as to give a magnified view of the distant object that is being examined. By means of such an instrument, the movements of the enemy can be followed without danger, the apparatus being generally of small size, and not attracting notice. Amongst the varieties of this instru- ment, is one whose use is readily seen by inspecting fig. 61, by which it seems to be perfectly possible to see with safety all that is going on outside the door of the house without being perceived. The line of the mirrors in this case is at right angles to that of the polemoscope in fig. 60. Amongst the different varieties of polemo- scope which have been invented, may be mentioned a re- flecting opera- glass, which was greatly used by the beaux and dandies of the last century. In the tube of this instrument was inserted an inclined mirror, which allowed the spectator to point his glass in quite a dif- ferent direction to that of the object he was really looking at. In fact, it was constructed somewhat on the same principle as the Herschellian or Newtonian telescope, and enabled the possessor, while apparently enjoying the play, to observe all that was going on in the boxes or pit of the theatre. Years ago, there was a little instrument of a similar kind, sold for a penny in the streets of London, which consisted of a morsel of looking-glass set at an angle, in a pill-box, and which gave the possessor the power of seeing all that was going on behind him. Persons who wear dark pre- servers are often in the habit of observing all that is 212 THE WONDERS OP OPTICS. going on behind their backs by the reflection seen in the corner of their glasses. Such are the principal optical recreations founded on the reflecting and refracting properties of mirrors and lenses. We shall end this chapter by appending to it the description of a few additional optical amusements that are quite within the reach of the amateur. If the reader is in possession of a concave mirror, it may be made the means of performing a number of amusing experiments. In front of it is placed a plaster head, a skull or any other object, mounted on wheels and running along a grooved platform, which is natu- rally kept perfectly concealed from the spectators. The mirror is slightly inclined, so as to reflect the image of the object at an angle to the observer's eye. By run- ning the cast backwards and forwards, it will have the appearance of advancing and retiring from the specta- tor in a very imposing manner. A dagger may be sub- stituted for the cast, and by being made to work up and down on a pivot, will have the appearance of striking at the spectator. We have already seen that an ex- periment of this sort had such an effect on Louis XIV. that he drew his sword to defend himself from his imaginary aggressor. There is another way of per- forming this trick, by suddenly illuminating the skull or dagger by means of a dark-coloured box containing a light, which may be made to throw its reflections on the object, by sliding it along a couple of wires. In the case of the dagger, however, the hinged arrange- ment will be found more effective. One of Robertson's tricks was called the "Magic Box," and he astonished a numerous party of visitors who were staying at a country house to which he had been invited. One of the gentlemen who was always boasting of his freedom from superstitious feelings of any kind, had had several arguments with Robertson on Fkj, 6i. — protection against ill-natured people. 213 OTHER OPTICAL ILLUSIONS. 215 the subject of apparitions, and the latter thought that he would at any rate surprise his strong-minded friend by an easy trick or two. He consequently chose as his confederate a lady to whom the gentleman had been paying great attention during the time of his visit. Robertson one evening mysteriously delivered a small box to him, which he was to place upon his toilet table, and unlock exactly at midnight. The gentleman did so, and what was his astonishment to see the face of the lady with whose charms he had been so deeply impressed suddenly spring out of the box. His look of terror and surprise was evidently too much for Robertson's confe- derate, who burst into a merry peal of laughter, leaving her admirer in a very disconcerted state. After all we have said on the subject of mirrors, it is not difficult to guess how this trick was performed. The box in question was painted black on the inside, and contained a concave mirror placed at an angle of 45°. The reflection of the lady, who was of course in the next room, was carried by means of several plane mirrors placed in boxes communicating with each other through the partition of the room, the head of the lady only be- ing strongly illuminated, the rest of her figure not ap- pearing by being kept quite dark. The figures reflected from smoke are extremely sur- prising. To perform such experiments a phantasmago- ria is necessary. The focus is so adjusted that the dis- tant image falls just above a brasier containing lighted charcoal. Everything being ready, a few grains of oli- banum or other gum are thrown on the coals, and the smoke that rises immediately affords a screen for the reflection of the images proceeding from the phantasma- goria. If the amateur is not the possessor of a magic lantern, a properly arranged concave mirror will answer almost the same purpose. 216 THE WONDERS OF OPTICS. CHAPTER IV. THE PROPERTIES OF MIRRORS. Almost every one in his younger days has possessed and broken that pretty instrument known as the ka- leidoscope. His researches into its construction no doubt taught him that it consisted of a cylindrical tube in tin or cardboard, with a moveable cap at one end and a small hole at the other. In the interior of the tube were found three long glasses, blackened on the back, placed at an angle, and kept in position by pieces of cork. The moveable cap was provided with two cir- cular pieces of glass, one ground and the other trans- parent, between which were placed a number of pieces of coloured glass. On holding the instrument up to the light and looking through the eye-hole, a beauti- fully coloured star was seen whose form and hue changed by simply shaking the tube. The kaleidoscope was invented by Sir David Brewster, and is exceedingly simple in principle. We all know that if a luminous object, such as a taper, is placed be- fore a mirror, it gives forth rays of light in all direc- tions. Amongst these luminous rays, those that fall on the surface of the mirror are, of course, reflected in such a manner that the angle of reflection is equal to the angle of incidence. Ir another mirror be placed at right angles to the first, and an object be put in the angle, the image of it will be multiplied four times. If the angle be diminished to 60°, six reflections will be seen, THE PROPERTIES OF MIRRORS. 217 and so on. A symmetrical figure is constantly obtained, forming in one case a cross composed of four similar portions ; in the other a triple star, the halves of each ray being similar. It is the symmetry of the figure that gives the pleasing effect. In the ordinary kaleidoscope the angle made by the reflecting surfaces is thirty de- grees, and a star of six rays is formed, the halves of each ray being alike. The figures formed in the ka- leidoscope are simply endless ; and if the space between the glasses in the moveable cap be filled with bits of opaque as well as transparent substances, the varieties of light and shade may be added to those of colour. It was at one time the fashion to copy the images formed in the kaleidoscope as paterns for room papers, muslins, curtains, shawls, and other similar fabrics, but thanks to the spread of artistic taste in this country the lecorative designer now relies more on his own talent than any aid he may receive from optical instruments. Plane mirrors, as we have seen, reflect objects up- right and symmetrical, reversing only the sides. Con- cave mirrors reverse them, and if they are not placed exactly in the proper focus, distort them by makingone portion appear smaller than the other ; while convex mirrors reflect them in an upright position, but also similarly slightly distorted. But when the mirror is not a portion of a sphere, like those whose properties we have been considering, the distortion is increased to so great an extent as to deform the object so that it is difficult to recognise its nature from its reflection. We all know the distortion that our face undergoes when reflected from the shining surface of a teapot or spoon, and the cylindrical mirrors that hang in the shop win- dows of many opticians are the source of much amuse- ment to the passers by, whose physiognomies are shown to them either lengthened to many times their natural size, or widened to an extent that is ludicrously hide- 218 THE WONDERS OF OPTICS. ous, according to the position in which the mirror is hung. Such distortions are known to opticians as ana- morphoses, from two Greek words signifying the de- struction of form ; and distorted drawings used to be sold at one time which when reflected from the surface of the cylindrical mirror, became perfectly symmetrical. Anamorphic drawings may be also made, which when looked at in the ordinary manner appear distorted, but when viewed from a particular point have their symme- try restored to them. With a little knowledge of draw- ing, it is not difficult to produce these in great variety. Suppose the portrait in fig. 62 to be divided horizon- tally and vertically by equidistant lines comprehended within the square abcd, A B . C D Fig. 62: Upon a second piece of paper draw the figure shown in fig. 63 in the following manner. Draw the horizon- tal line a b equal to A B (fig. 62,) and divide it into the same number of parts. Through the centre draw a perpendicular line to V, and cross it by a line e d parallel to a 6. Lastly, draw V S horizontal to e d. The length of the two lines e V and s v is quite arbitrary, but the longer you make the former in proportion to the THE PROPERTIES OF MIRRORS. 219 latter the greater will be the distortion of the drawing Now draw the lines v 1, v 2, v 3, and V 4, and join g to a. Wherever s a crosses the divisions 1, 2, e 3, 4, and 6, draw a horizontal line, parallel of course with a b. You will thus have a trapezium abed divided into as many spaces as the square A B c D in fig. 62, and it now remains to fill them in with similar portions of the figure. Thus, for instance, the nose is in the fourth vertical division, starting from the left, and in the third and fourth counting from the top ; in order, therefore, to make it occupy so lengthened a space it must be considerably distorted by the pencil. It will be readily seen also that the more numerous the spaces into which the square is divided, the easier it will be to draw the distorted picture. It is by this means that the anamorphosis shown in fig. 63 has been drawn. The next thing to do is to find the point of view from which we can see the figure in its natural propor- tions. This will be found to be at a distance above the point v equal to the line V S. In order to complete the experiment it is simply necessary to place the distorted picture in a horizontal position, and fix a piece of card- board vertically at the point v. If a hole be punched in it at a distance from v equal to S, and the drawing be looked at through it, the whole of the parts will fall into symmetry immediately. The experiment may be tried first with fig. 63, the hole being made rather large, and the eye placed at a distance of from 3 to 4 inches. It Avould be difficult, without having recourse to geometrical formulae, to explain how it happens that by placing the eye at a particular point the distorted lines of the drawing become symmetrical ; but perhaps a mechanical demonstration will help to make this difficult subject a little plainer. 220 THE WONDERS OF OPTICS. Draw in outline any figure upon a piece of cardboard, and make a series of pin-holes along the most prominent lines of the drawing, taking care that they are pretty a ± 2 & s s. I Fig. 63. — Anamorphosis. close together. Place the perforated card in a vertical position on a sheet of paper, so that the rays from a candle or lamp may fall, on the flat surface beneath. On looking at the luminous figure formed from the drawing, you will find that it is as much distorted as the lady's THE PROPERTIES OF MIRRORS. 221 head in fig. 63, and that the lower you place the candle the greater will be the deformity. You may if you please, trace the luminous figure on the paper, and the result will appear distorted when looked at in the ordi- nary manner, but symmetrical when viewed from the point at which the flame of the candle was placed. In the foregoing experiments we have spoken of the anamorphic drawings as being placed in a horizontal position, but they may be looked at just as well verti- cally, the card with the hole being in this instance hori- zontal. It is also not necessary that the point of sight (v, fig. 63) should be in the centre of the picture ; it may be placed at one side or the other, care being taken to draw all the divisional lines so that they meet at this particular spot. A few experiments with a can- dle and a perforated figure will soon show the student how to accomplish this. Anamorphoses by reflection may be prepared, if this principle is carried out, which appear a mass of con- fused lines until they are reflected in a cylindrical mir- ror. Formerly opticians were accustomed to construct anamorphoses which became symmetrical pictures when viewed in a conical mirror ; but the fashion for such toys appears to have gone out. Such drawings were extremely difficult to make, and the mirrors, having to be ground and polished with great care, were very ex- pensive. Some experimentalists have carried the subject so far that, by looking at the drawing of an object in par- ticular positions, it changed into quite a different sub- ject. In the cloister of an abbey that once existed in Paris, there were two anamorphoses of this kind. They were the work of a certain Father Niceron, who hats left behind him a treatise in Latin on optical wonders, entitled Thaumaturgus Opticus, which contains a long essay on anamorphoses. One of these pictures repre- 222 THE WONDERS OF OPTICS. eented St. John the Evangelist writing his Gospel; the other Mary Magdalene. When looked at in the ordi- nary manner, they appeared to be landscapes; but when the observer placed himself in a particular posi- tion, they changed into the figures we have mentioned. CHINESE SHADOWS. 228 CHAPTER V. CHINESE SHADOWS. While upon the subject of optical wonders, we should hardly be forgiven if we did not give a descrip- tion of the amusement known as Chinese shadows, or Fantocini. In the winter time it is difficult to pass through any of the large thoroughfares of London after nightfall, without seeing a crowd admiring the popular fantocini farces of the "Broken Bridge," or " Billy Button; " and although these dramatic exhibitions are not always free from vulgarity, they are received with vociferous applause by at least the younger portion of the audience. The apparatus for the exhibition of the fantocini is generally very simple. The screen on which they are shown is generally made of calico rendered semi-trans- parent with copal varnish, and the figures are cut out of cardboard. Frames containing landscapes and scenes of different kinds are also provided, which are cut out in the same material. The dramatis personce are gen- erally made with moveable limbs, which they throw about in the most unanatomical manner, and the showman is often endowed with ventriloquial talents of no mean or- der. This amusement is to be found in all parts of the world, from the Strand and Tottenham Court Road London, to the streets of Algiers and Java. A graphic writer in the Magasin Pittoresque gives a pleasant de- 224 THE WONDERS OF OPTICS. scnption of the fantocini, as exhibited at the Arabs' theatre in the Mohammedan quarter of the city of Al- giers. It was on the occasion of the feast of the Bai- ram, which immediately follows the termination of the Ramadan, or Mohammedan Lent. The theatre, which was the only one frequented by the Arab population, consisted simply of a long vaulted hall, without seats, boxes, or galleries ; but the audience, who had already been there some time, did not seem to regard the omis- sion as of any consequence, but had seated themselves on the ground with great coolness, chatting in whispers, and waiting patiently until the director should consider the place full enough to begin the performance. Half an hour elapsed, and the spectators still chatted on quite unconcernedly ; an hour, and yet there was no hissing or stamping of feet from the grave and patient spectators. At last they reached the maximum, and a boy came forward and blew out the few lamps with which the theatre was lighted, leaving them to smoulder away with a perfume that was certainly not Oriental in its character. First came the legend of the Seven Sleepers; then Scheherazade relating her bewitching stories to the Sultan. These were followed by Aladdin and the Wonderful Lamp, a story that is as popular in Algiers as it is in London or Paris ; the whole culmina- ting in a kind of burlesque, in which a great deal of gross fun was mixed up with a number of rebellious al- lusions. The devil, for instance, who is of course one of the members of the troupe, is portrayed as a French soldier, bearing a cross on his breast like an ancient Crusader. After him came Carhageuse, who is the buffoon of the Eastern stage, and who makes violent but unsuccessful love to a charming young Jewess. There was a poor barber who was raised to the dignity of grand vizier, his successor's head being cut off by the yataghan of the Oriental Jack Ketch, to the great de- CHINESE SHADOWS. 227 light of the people. Then a wretched Jew receives the bastinado, amidst vociferous applause, which increases still higher when the ears of an unhappy Giaour are cut off and thrown to the dogs. Throughout the piece, it is of course the Mussulman who always triumphs, like the French guards at the Cirque Imperiale, or the Brit- ish grenadiers at old Astley's. The performance con- cluded with a grand naval battle between the Moorish and Spanish fleets. The drum as usual served for can- non, there was a great deal of .smoke and confusion, and the Christian fleet gradually sank under the con- tinuous fire of the Mussulmans amidst the plaudits and bravos of the crowd. In Java, the subjects of the fantocini are generally taken from the native mythology. The screen on which the shadows are exhibited is ten or twelve feet long, and five feet high, and the figures are cut of thick leather, their limbs being moved by thin pieces of nearly transparent horn. In fig. 64 we see another kind of Chinese shadows, in which the lights of the figure are cut out. These pictures are perfectly unrecognisable as being even the basest imitation of any known form ; but when their shadows are thrown on the wall, the cut-out portions show us lights, whilst those that have been left form the shadows. On the Boulevard des Capucines, at Paris, there used to be a man who managed to pick up a good living by selling these candle shadows. Of course he used to carry on his trade of an evening, and with a strong lamp he would throw the shadows of his figures on the white walls of the houses, or the blind of a shop window, or even on the pavement. With a little care and ingenuity a number of these amusing cards may be easily designed. In showing them, care must be taken to choose the best distances between the light and the paper, and between this latter and the wall. 228 THE WONDERS OF OPTICS. If the card be placed too close to the wall, the resulting shadows will be too dark, and the outlines too sharp ; if, on the contrary, the light is placed too far off, the outlines become confused, and the proper effect is lost. Shadows have been applied before now to the propa- gation of seditious ideas. " In 1817," says an esteemed French author, u one winter's night we were all sitting round the table listening to my father, who was reading aloud an interesting book of the period, when a friend of our family, who had been formerly an officer of the Empire, entered the room. He was a serious, upright, soldierly man, and wore his coat buttoned up to his chin. He had hardly replied to our salutations, when he drew a chair to the table, and made a sign with his hands and eyes that plainly indicated silence and dis- cretion. There was something in the expression of his countenance that seemed to show that he had something mysterious in store for us, and we fully expected to hear some extraordinary news, or to see him bring out a Bonapartist pamphlet of more than usual importance. Our surprise was consequently great when we saw him slowly unscrew the top of his cane, which was turned out of boxwood, and presented nothing very remarkable either in form or material. He, however, took up a copybook which was lying on the table, placed it at a certain distance from the lamp, and then laid upon it the little piece of turned boxwood. At first we noticed nothing at all extraordinary, and he smiled at our want of intelligence, until at last my youngest brother cried out suddenly, 6 Look ! there's the head of Napoleon V and truly enough, we found, on looking more attentively at the shadows of the turned knob of the cane, that their profile was that of the great exile, most correctly and clearly portrayed. The old captain's face lighted up at the sight, and the tears came into his eyes. ' We shall CHINESE SHADOWS. 229 see him again/ he murmured in a low voice, and he hummed the burden of a Bonapartist song then in vogue. During the rest of the evening he was very Cane. Fig. 65 — Seditous Toys. lively, and proved to us most conclusively, that before six months the Crrande Armee would be revenged for 230 THE WONDERS OF OPTICS. their defeat at Waterloo. Some weeks after, there was hardly a soldier in the town that did not possess a stick or a tobacco-pipe stopper, turned in this fashion, but one day a panic seized everybody, and the canes and pipe stoppers were all burnt. ,, Fig. 65 represents historic heads cut in this way. During the Shakespeare Tercentenary excitement, a London turner made quite a little fortune by making heads of the great poet on the same principle. P0LY0RAMA— DISSOLVING VIEWS — DIORAMA. 231 CHAPTER VI. POLYORAMA-^-DISSOLVING VIEWS — DIORAMA. The description of the polyorama naturally follows that of the phantasmagoria, being a practical applica- tion of precisely the same principles. In the case of the polyorama, however, two or even more lanterns of the best construction, are used. There are therefore two sets of lenses identical in every particular, placed side by side, in the same line, the foci of both being adjusted for the same spot, so that the images refracted from each may superpose each other without difficulty. In each instrument there are the same pictures, but they differ in certain particulars, as we shall see pres- ently. In the phantascopes shown in figs. 52 and 54 there are two sets of lenses ; the first carries a glass bearing the image of a skeleton in a winding sheet, while on the glass belonging to the second a naked skeleton is por- trayed. If, therefore, at a given instant the first lan- tern is shut off, the spectators see the winding sheet torn, as it were, suddenly from the spectre before them. The first lantern being turned on once more, the skele- ton is instantly reclothed in its hideous garb. It is of course not necessary always to choose such horrible subjects for representation, as it is possible to produce changes of a much more agreeable nature. For instance, a volcano may be depicted during its 232 THE WONDERS OF OPTICS. tranquillity, with the sun shining on its verdant sides, and surmounted with a gently rising wreath of smoke. Then it may be shown at night, with its crater vomiting flames and red-hot stones, while streams of lava are flowing beneath. By proper mechanism, one lantern may be gradually shut and the other as gradually opened, producing an effect that appears perfectly natural, from the gentle change which takes place. Daylight, twilight, and moonlight effects may be easily made to succeed each other in their proper order, and the most opposite scenes may be made to change each other by proper appliances. Those who have seen the dissolving views at the Polytechnic, know what effects are produced by this very simple means. A virgin forest changes to a crowded church, which in turn dis- solves into a scene on the Alps. The diorama, properly so called, invented by the il- lustrious Daguerre, differs completely in principle from the apparatus we have just been describing. As its etymology indicates, the pictures shown are seen through. As in the case of the polyorama, there are two different effects painted upon the cloth, which are brought out by a double system of illumination. Fig. 66 will show the way in which these changes are managed. The large picture, which is hanging verti- cally, is painted both in front and behind. The front is illuminated by reflection from a semi-transparent screen placed over it, which receives the light of the floor above. The back is lighted from the windows be- hind, which are provided with blinds to regulate the am>unt of light. The effects produced by the diorama were truly marvellous, and Daguerre had a special ta- lent for this kind of painting. His famous Midnight Mass, which was exhibited at the Regent's Park, was one of the most renowned of his works. The scene first represented a dark, empty church, feebly lighted by a POLYORAMA — DISSOLVING VIEWS — DIORAMA. 235 small altar lamp, but gradually the lights appeared here and there, worshippers congregated in front of the al- tar, filling the nave and aisles. In Paris the same scene was exhibited, representing the interior of the Church of St. Germain l'Auxerrois with such perfect reality, that a countryman actually threw a halfpenny against the painted canvas, to see whether he were really in a church or not. The next scene represented the destruction of the village of Goldau, near Lucerne, by a landslip. First there appeared a smiling fertile valley, its sides crowned with verdure ; a storm gradually rose, the rain fell, the wind blew, the lightnings flashed, and the thunder rolled in the distance. Darkness at last closed in, and when the sun once more rose over the valley, nothing was to be seen but a mass of fallen rocks. THE WONDERS OF OPTICS, CHAPTER VII. THE STEREOSCOPE. Having devoted so much space in the preceding chapters to optical amusements of a purely recreative character, it is only right that we should now say a few words on certain instruments of a less frivolous character than those we have lately been considering, and which deserve at our hands the most serious attention. We shall, therefore, in the present chapter, speak of an in- genious instrument which serves to show in relief the images of objects depicted on a flat surface. We have already seen, that although we have two eyes, provided with lenses and screens by means of which the images of things around us are formed, we only perceive a single object ; and the student has no doubt long since wondered why nature has bestowed two eyes upon us, when only one would have apparently served the same purpose. This question was for a long time a complete puzzle to philosophers, and it was not until Professor Wheatstone made his experiments on binocular vision in 1838, that the matter received a satisfactory explanation. He showed that each eye receives a dif- ferent impression of any object upon the retina, and that it is in consequence of the union of these slightly dis- similar images that the sensation of relief is experienced.' A one-eyed man or a Cyclops would only partially perceive relief in the objects presented to his view, in THE STEREOSCOPE. 237 consequence of a single image being sent to his brain. He would, no doubt, after examining the things he saw with his hands, know they were solid, and generally see them so ; but if a new object were presented to his view he would have some difficulty in knowing whether it had a flat surface or not. FA Fig. 67. The principle of binocular vision may be explained as follows : If a playing die, such as is represented in fig. 67, be held out at arm's length in the position indi- cated in the figure, and looked at first with the left eye and then with the right, we shall find that in the first case we see a little of the three dots on the left-hand side, and in the second we lose sight of the three dots and see a little of the single one on the right-hand side. The images seen by each eye are, therefore, slightly dis- similar, and it stands to reason that, if by any means we can combine two slightly dissimilar flat pictures of a solid object, we shall see it in relief. This was proved practically by Professor Wheatstone, who constructed an instrument capable of effecting the desired union, and which has since been called the stereoscope, from two Greek words signifying 4 to see solid. ' The instrument remained for a long time fallow, so to speak, from the difficulty of drawing two pictures that should be iden- tical in size and details, although dissimilar in the arrangement of their perspective. It was, therefore, not until photography enabled us to do this with the greatest 238 THE WONDERS OF OPTICS. ease and exactitude that the stereoscope became com- mon. The instrument first devised by Professor Wheat- stone, was what is termed a reflecting stereoscope, and was expensive to make and cumbrous to use. It was Fig. 68.— Stereoscope. modified by Sir David Brewster, by the substitution of prisms for reflectors, and was thus made cheaper and more portable. The refracting form of stereoscope is so familiar to most people, that it really needs no description. It will only be necessary to mention that the prisms used in the eye-pieces are made by cutting a double convex lens in two, and reversing the halves. They are so placed that the centre of each prism is just in the centre of each eye ; but as the eyes of "different people vary in distance, an arrangement is generally added so that the eye-pieces may slide from side to side. Being cut from lenses, the prisms have a magnifying power; consequently other means are provided for sliding them up and down to suit the length of focus in different eyes. In fig. 69 we can follow the path of the rays pro- ceeding from each picture, and reach the eyes appa- rently from a spot exactly between the two. In the reflecting stereoscope two mirrors are joined THE STEREOSCOPE. 239 together at right angles to each other, the two pictures being placed at each side, at a distance corresponding to their size. The reflecting instrument, although not Fig. 69.— The Principle of the Refracting Stereoscope. so portable, is in some sort superior to the other, inasmuch as pictures of any size can be seen by it, whilst in the smaller instrument the size of the photo- graph is limited by the distance at which the eyes are placed. It should be mentioned, that no optical instrument of any kind is absolutely necessary to obtain a stereoscopic effect from two suitable drawings or photographs, as it is quite possible by a little management of the eyes to cause the two images to combine with each other. Re- ferring again to fig. 67, it will be perceived that the two figures of the dice are about an inch and a half from each other. Holding the book at about ten inches from the eye, they are viewed by squinting strongly until the right eye looks at the left die, and the left eye at the right. This may be also done by converging the eyes on a point beyond the centre of the figure, which maybe easily done by looking at a point midway between the two. In both cases the images at first appear dou- bled, and we see four dice, but a little practice will soon enable you to cause the two inside images to coalesce, 240 THE WONDERS OF OPTICS. and so give the effect of relief. It is true that even then three images are seen, but the eye soon grows accus- tomed to neglect them altogether. This habit is a very pleasant acquirement for the London flaneur, who can thus see in perfection the numberless stereoscopic views now shown in our shop-windows without the intervention of an instrument of any kind. The method of photographing subjects for the stereo- scope is very simple, and consists in taking two views of the object to be depicted, from two different points. According to the distance of these points from each other, so will the resulting pictures appear in greater or less relief. This is readily seen in some stereoscopic portraits which have been taken at a large angle, and consequently show such increased relief as to produce distortion. Theoretically, the interval of the two points of view ought to be two inches and a half, that being the average distance between the two eyes ; but in practice it is better to increase it in the case of portraits or other near objects to about twelve inches, and in that of view's to even several feet. Brewster's original rule for taking stereoscopic photographs, was to place the cameras one foot apart for every twenty-five feet of dis- tance. The beautiful stereoscopic pictures of the moon photographed by Mr. Warren de la Rue were taken at more than 1,000 miles' distance, in order to obtain the necessary relief. The principle of the stereoscope has received many useful applications in the way of book illustrations, art teaching, and anatomical demonstra- tion, and has thus gained a position among philoso- phical instruments that it did not at first possess. A combination of the principles of the phenakisti- scope (fig. 4) and stereoscope, has resulted in the invention of an instrument called the stereotrope. A number of binocular photographs of some object in motion — a steam-engine, for instance — are taken when THE STEREOSCOPE. 24.1 the moving parts are in different positions, and mounted on two revolving discs, the images being combined by means of a pair of semi-lenses, as in the ordinary re- fracting stereoscope. "We cannot leave this subject without describing the pseudoscope, also the invention of Professor Wheat- stone. If a stereoscopic pair of photographs of some solid body — a ball, for instance — are mounted the re- verse way, that is to say, if the picture intended to be looked at by the right eye is placed on the left, the relief of the object will be reversed, and the ball will appear as a hollow hemisphere. If, therefore, we can by means of lenses or prisms cause the image of any natural obje t, as seen by the right eye, to be conveyed to the left, and vice versa, we shall see the relief reversed. A conical cap will appear in relief as a cone, a globe will look like a hollow sphere, and the human face will take the semblance of the inside of a mask. The same deception may be effected by looking at a seal through a short-focused lens, so that the image shall seem re- versed. In this case, the light coming apparently from the wrong side, and shining on the parts in relief, gives them the appearance of being hollow. An intaglio will, of course, appear in relief when so looked at. Photographs of gems and bas-reliefs will also present a pseudoscopic appearance, if looked at in a light coming from the opposite side to that in which they were taken. The same appearance may be seen some- times in wall papers having patterns painted in strong relief. 242 THE WONDERS OF OPTICS. CHAPTER VIII. THE CAMERA OBSCURA AND CAMERA LUCIDA. The construction of the camera obscura is found- ed on the fact that the rays of light, when col- lected into a point either by being passed through a small hole or a converging lens, form an image of the objects from which they proceed at the point of meeting. This may be readily tried by piercing the shutter of a room with a small hole, and holding a piece of paper within a short distance of it. It will be noticed that the smaller the hole the more distant will be the image formed. The first person who observed this fact was John Baptist Porta, an Italian philosopher who lived in the latter part of the seventeenth century. He noticed that when a screen was placed opposite a small hole in the shutter of his room, the objects outside were depicted on it in a reversed position with moderate dis- tinctness ; but that when a biconvex lens was placed over the hole, the picture was rendered much more distinct. This was the first attempt at the formation of the camera obscura, an instrument that has since be- stowed such incalculable benefits on humanity. The shape of the images so formed is independent of the shape of the opening, which, as long as it is suffi- ciently small, may be square, oval, or triangular. This may be easily seen when the sun shines through the intervals between the leaves of a shady avenue or bower THE CAMERA OBSCURA AND CAMERA LUCIDA. 245 of trees. The image of the sun as a circular patch of light is seen scattered over the surface of the ground, although the accidental intervals formed by the leaves above were of a thousand different shapes. These images at the time of an eclipse of the sun are very surprising, taking, as they do, the form of a crescent, more or less large according to the magnitude of the eclipse. This property possessed by the rays of light, of de- picting on a. screen the forms and colours of the objects from which they proceed when passed through a small aperture or a lens, is taken advantage of in most places famous for their natural scenery. The apparatus em- ployed for this purpose is comparatively simple, consist- ing merely of a dark wooden hut, with a whitened table in the centre, and a mirror and lens in the apex of the roof. In fig. 70 we have a section of a camera obscura of this kind. The mirror and lens at the top of the apparatus are made to revolve, so as to bring every part of the landscape into view in turn. A camera obscura in a position commanding a. view of moving objects, such as ships sailing to and fro, or the busy streets of a populous town, is an unending source of amusement, and may be easily and cheaply constructed. The camera obscura has been much utilized for taking hasty but exact sketches of various places. For this purpose it is made very light, and mounted on three legs carrying at their junction a flat table, whereon is placed the paper to receive the drawing. The tripod is cov- ered with a black curtain, which, falling over the artist, effectually excludes all the rays of light except those which pass through the lens and are reflected downwards by the mirror. In the better kind of apparatus the mirror is replaced by a prism, which throws a clearer image than a mirror upon the screen. It is on these properties of the camera obscura that 246 THE WONDERS OF OPTICS. the art of photography was founded. Everybody who saw the beautiful images formed by this instrument was struck with the idea that by some means or other they could be fixed on paper. After numberless attempts the long-wished-for goal was at length arrived at ; and now optics, aided by chemistry, is enabled to depict for us natural objects of every kind, from the distorted limb of the hospital patient to the beautiful forms of the queens and empresses of the world — from the tiniest animalcule to the great sun itself, who is compelled by the might of science to paint his own portrait for us with all his faults and imperfections. The lenses used for photographic purposes have only reached their present state of perfection after ceaseless labours of the philosophers and opticians of all coun- tries. At first only a single lens was used, but it was found that the rays which exercised a chemical action did not meet in the same point as the rays of light, for it must be remembered that it is not the light we see that acts upon the substances used in photography, but another influence, known as actinism. It was also found that a single lens would not give a flat picture when the whole of its apenure was used, the edges of the image being always blurred and indistinct. This latter defect was found to be partially obviated by de- creasing the opening, but this remedy shut off the light and prolonged the process. Gradually these two de- fects were removed, and now every photographer, no matter how humble, is possessed of a lens capable of taking a clear picture, every detail of which is perfectly distinct and faithful. The camera lucida bears a great analogy to the ca- mera obscura in the purpose for which it is used, though not in the principle on which it is constructed. It is employed, like the preceding instrument, for obtaining faithful copies of a landscape, a building, or even of an- THE CAMERA OBSCURA AND CAMERA LUCIDA. 247 other drawing. It was invented by Dr. Wollaston, in 1804, and consists of a little four-sided prism, of which fig. 71 is a section. Fig. 71.— Section of Camera Lucida. The angle at A is a right angle ; the angle B measures 67J°, the angle c 135°, and the angle D is, of course, equal to B. It is mounted on a sliding foot, so that it may be raised or lowered at will, or turned in a hori- zontal direction. The path of the rays in this case is easy to follow, the object to be copied being placed at L, and the eye at I. On looking downwards the image of the object to be drawn is seen on the paper; and if the eye is placed so that the edge of the prism will just cut the pupil in two, the paper and pencil will be seen at the same time. It will be seen from the diagram, that the rays proceeding from L strike on the surface A B at right angles, and, being then reflected from C B, pass upwards again to point E. The direction of the rays is in reality a little more complicated than this. In the case of distant objects it is impossible to see both the object and the pencil at the same time; a lens is sometimes introduced at I to modify this defect. The original instrument has also been modified by the intro- .248 THE WONDERS OF OPTICS. duction of a triangular prism, in conjunction with plates of coloured glass, but the difficulty of rendering the image and the paper of the same strength is very grea t. The instrument is also hard to use, from the additional difficulty of always keeping the head in the same posi- tion, for the least movement from left or right is suffi- cient to throw the whole drawing out. A simple camera lucida may be made out of a small piece of looking-glass, mounted at an angle of 45°, or half-way between the horizontal and the perpendicular. If this be turned towards the drawing or view to be copied, and the left eye applied to the mirror, the image of the object will be seen on the paper below, and the pencil may be guided with the right. The proper use of this simple little instrument depends in a great mea- sure upon the focus of each eye being the same. The light falling on the paper, too, requires very careful adjusting, otherwise the brighter object will eclipse the other. It is a good plan, too, to whiten the pencil or pen used, so that it may not so easily be lost when draw- ing the brighter parts of the object. We have seen excellent drawings made from plants by means of a little instrument of this kind, which simply consisted of a piece of looking-glass inserted in a cork stuck in a glass bottle. THE SPECTROSCOPE. 249 CHAPTER IX. THE SPECTROSCOPE. We now come to speak of an instrument which may fairly rank, after the telescope and microscope, as one of the most wonderful discoveries of modern optical science. By its means we have not only discovered four new elementary bodies, which are found in certain minerals in inconceivably small quantities, but we have also determined the chemical composition of some of the remotest stars and nebulae. In 1701 Newton discovered that if an ordinary ray of white light was admitted through a small hole into a dark chamber, and thence passed through a triangular prism, it became decomposed into a coloured band, known as the solar spectrum. As we have already ex- plained that this decomposition is caused by the different coloured rays that make up white light being bent un- equally by the action of the prism, we trust the follow- ing explanations will be readily understood. In 1802 Dr. Wollaston, an English philosopher, discovered that by using a narrow slit, instead of a round hole, the re- sulting spectrum was no longer continuous, but was divided at intervals by dark lines extending across it in a direction parallel to the edges of the prism. These line's attracted considerable attention at the time, but it was not until 1815, that Fraunhofer, an optician of Munich, investigated them with accuracy. He mapped 250 THE WONDERS OF OPTICS. and counted no less than six hundred of them, identify- ing eight of the most conspicuous by the first eight letters of the alphabet. Their positions are as follow : — The designations of these lines have been retained to the present day, and they have been named after the Munich philosopher, being known as Fraunhofer's lines. They are to be seen in all parts of the spectrum, and increase in number and fineness according as the width of the slit through which the light passes is diminished. It may be asked, how it happens that they increase in proportion to the narrowness of the aperture admitting the light ? A little consideration will soon show the reason of this. When a beam of light is passed through a hole of, let us say, the eighth of an inch in diameter and decom- posed by a prism, the spectrum so produced is imperfect, inasmuch as an infinite number of spectra are thus su- perposed, and for this reason, that the rays of light entering on the right side of the aperture will give a spectrum falling in a different place to that formed by the rays entering on the left. In order, therefore, to diminish the confusion caused by the superposition of a number of spectra, the aperture ought to be reduced to a narrow slit. When the thin slice of light passing through the slit is decomposed by the prism, we find that not only is the purity of the colours greatly in- creased, but the lines in question make their appearance more or less in all parts of the coloured band. These lines are very unequally distributed, some being crowded together in masses, while others are extremely faint, and are separated by large intervals. Their A. Beginning of red. B. Middle of red. 0. Beginning of orange. D. Middle of yellow. E. Middle of green. F. Beginning of blue. G. Middle of indigo. H. Middle of violet. THE SPECTROSCOPE. 251 position is well marked and determined, no matter from what source we obtain our beam of sunlight. Whether the spectrum be produced from the sun itself, or from the reflected light proceeding from the moon or planets, they are still found in the same place ; only that in the latter case they are not so numerous, on account of the light being much fainter. For many years the cause of these lines remained a complete mystery, and it was not until Bunsen and Kirchhoff undertook their investigation that a satisfactory explanation of their origin was ar- rived at. In order to explain this, we must consider briefly the properties of the spectra of flames, and other luminous bodies. If, instead of the light of the sun, we examine pris- matically the light given off by an incandescent body, such as a white-hot piece of platinum, we shall find that the lines seen in the solar spectrum are absent, and that we have a continuous band of coloured light quite unin- terrupted by dark spaces or bands. The same absence of lines is seen in the spectra of the electric light and the flame of an ordinary candle, the light in each of these cases being produced by particles of carbon in a state of vivid incandescence. But if we examine the flame of incandescent gases, we shall find a spectrum of an entirely new kind. Thus if we examine an ordinaiy gaslight through a slit with a prism, we shall obtain a continuous spectrum, in consequence of the luminous portion of the flame consisting of solid carbon in a state of incandescence ; but if we turn down the flame, so as to lessen the amount of carbon to be burned, we shall find the whole of that body is converted into feebly lumi- nous gas, giving off a faint reddish blue light. If we now again examine it in the same manner, we shall fii.d that the spectrum produced consists of black spaces, here and there crossed by a few faint coloured lines or bands. The reason of this is obvious : in the faint flame 252 THE WONDERS OF OPTICS. caused by the carbon and hydrogen in a state of lumi- nous vapour, which only have a few of the colours of the spectrum, which, when passed through the prism, fall into their proper places. All substances with which we are acquainted are capable of being converted into lu- minous vapour by means of heat, and when thus burnt produce Hamas of more or less faint luminosity, gene- rally characteristically coloured. A piece of soda inserted in the wick of a spirit lamp gives a yellow tinge to the flame ; a morsel of saltpetre (nitrate of potash) or nitrate of strontia will give a purple and crimson tint respectively. These hues are caused by the metals sodium, potassium, and strontium contained in these salts being converted into luminous vapour. On ana- lyzing these coloured flames with a prism, as before, we should find in the case of the soda a single broad yellow line, situated just in the middle of the yellow portion of the spectrum, the rest of the space where the spectrum should be being perfectly dark. The reason of this is pretty simple. Sodium burns with a pure yellow flame, consequently when passed through a prism it cannot split into any other colours, but takes its place in the position belonging to yellow of that particular hue. Were it a little more orange or green in tint, it would take its place nearer to the red or violet end of the spectrum. The light from saltpetre, which contains potassium may next be examined. It will be found to tinge the flame with the spirit-lamp of a beautiful purple. We can almost guess what will happen when this flame is submitted to the action of the prism. We shall find that the purple light emitted will split into red and violet, which will immediately arrange themselves in their proper positions according to their hues. If in like manner we substitute nitrate of strontia for salt- petre, we shall get a splendid crimson flame which if? decomposed by the prism into red, orange, or blue. THE SPECTROSCOPE. 253 On submitting the compounds of the other elements to the same tests, we shall find that each of them, when converted into luminous gas, is capable of producing coloured lines of various kinds when the light of their flames is passed through a prism. If, therefore, w T e had a number of salts of whose composition we were igno- rant, all we need do is to burn them in a spirit-lamp, and by the number and position in the lines of their spectra we should be able to tell immediately of what they were composed. - The spectra of nearly all the elements capable of being connected with luminous gas have been determined with great accuracy. Perhaps the number and position of the lines of a few spectra will be interesting to the student. Sodium. — This is the metallic base of soda salts, and gives a double bright yellow line in the middle of the yellow. Potassium. — The base of the various salts of potash. It gives one line in the extreme red, one in the middle of the red, one in the violet, and a peculiar glow in the centre of the spectrum. Strontium. — The base of the strontia salts, of which the nitrate is used as the principal ingredient in the red fire of the theatres. It gives a group of lines in the red and orange, and a beautiful blue one in the middle of the blue. Barium. — The base of the baryta salts, one of which is used in making green fire. It gives several strong lines in the green, and a few in the red, orange, and yellow. After the position of the spectral lines of most of the elements had been discovered, Messrs. Bunsen and KirchhofF were one day examining the saline deposit of a spring which issues from the earth near Durkheim, in 254 THE WONDERS OF OPTICS. the Palatinate, and were surprised to find that a blue line belonging to no known metal made its appearance In addition to the potassium, sodium, and other lines produced by the saline ingredients of the water. These philosophers immediately concluded that the unknown line was caused by an unknown metal, and they at once set to work to obtain a larger quantity of the saline residue from the spring. They evaporated down no less than forty tons of water, and succeeded in isolating the new substance, which turned out to be a metal resem- bling potassium. While examining the residue more carefully, a new, dark red line, beyond that belonging to potassium, was discovered, pointing to the existence of a second new element, which was also afterwards ob- tained in the pure state. These two new metals, which closely resemble potassium in their properties, were named in accordance with the lines given by them when converted into luminous gas. The first was called cae- sium, from ccesius, Lat. light blue; and the other, rubi- dium, from rubidus, Lat. dark red. Since the publica- tion of MM. Bunsen and KirchhofPs experiments, these two elements have been found in comparatively large quantities in various minerals, and these properties have been closely studied. Spectrum analysis has yielded us two more new metals since first these philosophers applied the prism to the determination of the chemical composition of various bodies. Mr. W. Crookes, F.R.S., an English chemist of eminence, while examining the flame of a deposit obtained during the manufacture of sulphuric acid from a certain sulphur mineral found in the Hartz mountains, perceived a brilliant green line with which he was previously unacquainted, which quickly flashed into view, and then disappeared. After numerous ex- periments on various other minerals (for the deposit he had first experimented upon only yielded him a few grains THE SPECTROSCOPE. 255 of the new body), Mr. Crookes succeeded in discovering a comparatively large quantity of it in a sulphur mineral found in Belgium. The new element was found to be a heavy metal, closely resembling lead in its properties. It was named by the discoverer, thallium, from the Greek word thallos, a green twig, from the brilliancy of the single green line that indicates its presence. In like manner, Messrs. Reich and Eichter have discovered a fourth new metal, which has been named indium, from its principal lines being found in the centre of the indi- go of the spectrum. The delicacy of spectrum analysis may be imagined from the fact that a quantity of sodium amounting to less than the two-millionth of a grain can be detected by its means. Indeed, it has taught us that sodium in one form or other exists almost everywhere. This mode of analysis is only serviceable to indicate the composi- tion of any salt or other substance, the quantities of the different elements found by its use having no influence on the appearances brought out by the prism. Thus, asubstance which has only been contaminated with sodium from being handled by warm fingers, will show the yel- low bands as strongly as it it contained a large propor- tion of that metal. For ordinary experiments in spectrum analysis the apparatus used is very simple. It consists of a tube with a fine slit at one end, and a convex lens at the other, for concentrating the light from the coloured flame upon the centre of the prism. After the light passes through the prism, it is examined by a small telescope of low magnifying power. The lamp used may be either a spirit-lamp or a colourless gas flame into which the substance to be examined is introduced upon a platinum wire. We now come to another very important discovery, made by means of our prism and narrow slit — the 256 THE WONDERS OF OPTICS. determination of the composition of the photosphere or mass of luminous vapour surrounding the body of the sun. A simple experiment will show how this brilliant dis- covery was arrived at. The light of a candle or other flame containing incandescent solid matter is passed through the spectroscope, and is found to decompose into a continuous spectrum, uninterrupted by dark lines. Between the light and the slit a spirit-lamp is placed, but no difference in the appearance of the spectrum is perceived. Introduce, however, the smallest portion of a soda salt into the non-luminous flame of the second- lamp, and a broad black line is immediately seen, cross- ing the middle of the yellow portion of the band of co- lour. Remove the sodium flame and the band disap- pears ; but do the same with the lamp producing the spectrum, and the spectrum of course disappears, and the dark band caused by the sodium flame is changed to the yellow line produced by that metal. The same experiments may be tried with potassium, strontium, and other metals ; and we shall always find that when a coloured flame is introduced between an incandescent solid and its continuous spectrum, it produces a series of black lines corresponding to the substances by which it is coloured. Thallium, in like manner, would give a black band in the middle of the green, and indium a similar one in the indigo. (Fig. 6, Frontispiece.) The exaci; position of the black band in the middle of the yellow is shown in the coloured figurq of the spec- trum so beautifully printed in the frontispiece of this book, and it has been found to correspond exactly with the dark line D of the solar spectrum. The inference from this fact is obvious. The incandescent portion of the sun gives off* light corresponding in its properties to that emitted by the solid matter contained in the candle flame, but the photosphere containing the vapour of so- THE SPECTROSCOPE. 257 dium cuts off that portion corresponding to the sodium line. Accurate measurements prove that numberless other lines occurring in the solar spectrum are due to the vapours of other well known metals existing on the earth. Amongst these may be mentioned potassium, calcium (the base of lime), iron, nickel, chromium, and several others. This discovery with regard to the sun has resulted in the spectral examination of a large num- ber of the fixed stars and nebulae. For centuries the fixed stars refused to answer all questions put to them by mortals. The telescope showed them merely as bright points. Their nature and origin remained a beautiful mystery, until Dr. Miller, Mr. Huggins, Father Secchi, and a few other philosophers interrogated them in a manner that could not fail to draw forth an answer. They brought their light within range of their prisms, and forthwith they declared themselves to be suns like our own. It is true that before this they were looked on by most astronomers as bodies analogous to our own sun, but it was only reasoning from analogy, after all ; but we are now able to assert with all the certainty that is compatible with human fallibility that many of these heavenly bodies are possessed of an incandescent centre, surrounded by a photosphere or envelope of gaseous matter in a luminous condition. It would be impossible to give a list of all the stars that have been examined up to the present time; the composition of the photospheres of a few must therefore suffice. It is singular that the elements hitherto discovered in the stars are those which are more or less abundant on the earth. Amongst them we may name hydrogen, nitro> gen, sodium, magnesium, barium, iron, antimony, bis- muth, tellurium, and mercury. The bright star in the constellation of Orion known as Betelgeux is one of the most singular in composition, the lines of its spectrum indicating the absence of hydrogen. If, as Messrs. R 258 THE WONDERS OF OPTICS. Huggins and Miller suggest, the worlds revolving round this star are also deficient in this element, they would be without water, like our moon. Upon a very clear night it may be noticed that the stars are not all of the same colour, but that many of them appear to be of a ruddy or yellowish tint. The cause of this is plainly seen when they are submitted to spectral analysis. Thus, Sirius, which is a brilliant white star, shows but three dark lines, while one of the stars in the constellation of Hercules shows several groups of bands in the red, blue, and green portions of its spectrum, fully accounting for its orange tint. The double star /3 Cygni is a very beautiful example of the distribution of colour between two members of a stellar group. One star shows a strong spectrum with the blue and violet portions almost totally blotted out, while its companion is similarly circumstanced with re- spect to the yellow and orange portions of its spectrum. The colour of one is consequently orange, while the other is of a delicate blue. If these stars are the principal members of a system, the alternation of blue and orange days must be indeed a singular phenomenon to those who inhabit their planets. In some of the stars lines have been discovered which do not possess any equivalent amongst those pro- duced by terrestrial matter ; they consequently contain elements of which we know nothing ; at the same time, however, it has been found that terrestrial elements exist in some of the remote nebulse, which are so dis- tant that their light takes many thousands of years to reach our earth. Spectrum analysis has decided the grand question of the physical composition of the nebulae. Those bodies were supposed, with some reason, to be aggregations of stars, like our Milky Way, which only required tele- scopes of sufficient power to resolve them. That they THE SPECTROSCOPE. 259 partly consist of gaseous matter in a luminous condition is evidenced by their showing a series of bright lines in the spectroscope, exactly like those produced by terres- trial gases. Their light is therefore not emitted by a solid or liquid incandescent body, but by a glowing gas. The lines mentioned by Messrs. Huggins and Miller showed that the nebula in the sword-handle of Orion consists of hydrogen and nitrogen in a state of luminous incandescence. Not the slightest trace of a continuous spectrum can be detected in the light emanating from this body ; consequently, according to present hypo- theses, it contains no solid matter at all. A number of other nebulae have given similar results. There are numerous star clusters which, unlike the true nebulae, give continuous spectra when their light is submitted to the action of the prism. Of these may be specially mentioned the great clusters in Andromeda and Hercules, which give continuous spectra, inter- rupted by dark bands on the red and orange. The light thrown by these experiments upon the nebular hypotheses of Sir William Herschel, who considered that true nebulae consisted of the primordial gaseous matter out of which suns and stars have been elabo- rated, is very great, and will be appreciated even by those whose knowledge of astronomy is small. Spectral analysis has also been the means of our wit- nessing a celestial conflagration, and understanding the cause of this marvellous event. It is well known to most people that from time to time stars have suddenly burst upon us, and have almost as suddenly disappeared. The theories advanced to account for these singular celestial visitors, have been more numerous than satis- factory. In May 1866, a star of the second magni- tude suddenly burst forth in the Northern Crown, and was almost immediately noticed by Mr. Huggins who brought every power of prism and telescope to bear 260 THE WONDERS OF OPTICS. upon this extraordinary celestial phenomenon. He found the spectrum of the star to consist of two distinct spectra, one being formed by four bright lines, the ttther analogous to the spectra of the sun and stars. Consequently two kinds of light were given off by this star ; one forming a series of bright lines indicative of luminous gas, the other consisting of a continuous spectrum, crossed by dark lines, showing the existence of a solid body in a state of incandescence, surrounded by a photosphere of luminous vapours. Two of the bright lines undoubtedly showed the presence of hydrogen in a state of illumination, the great bright- ness of the lines indicating that the burning gas was hotter than the photosphere. These facts taken in conjunction with the suddenness of the outburst in the star, and its immediate decline in brightness from the second down to the eighth magnitude in twelve days, suggest the startling speculation that the star had be- come suddenly wrapped in the flames of burning hydrogen, consequent possibly on some violent con- vulsion in the interior of the star having set free enormous quantities of this gas. As the free hydrogen became exhausted, the spectrum showing the bright lines gradually waned until the star decreased in bril- liancy. It must not be forgotten that the event seen by Mr. Huggins occurred many years ago, and that the light emitted by this marvellous celestial convulsion has been travelling to us ever since. Comets and meteors have been submitted to the test of spectral analysis. The former erratic visitors have been but few and small since stellar spectrum analysis has been perfected. In January 1866, Mr. Huggins brought his apparatus to bear upon a small comet, which gave a somewhat unexpected result. When the object was viewed in the spectroscope, two spectra were distinguishable — a very faint continuous spectrum of THE SPECTROSCOPE. 261 the tail, showing that it reflected solar light, and a bright space towards the centre of the spectrum, indi- cating that the nucleous was self-luminous and gaseous. Mr. Alexander Herschel — the nephew and the grand- son of Sir John and Sir William Herschel — has recently succeeded in obtaining indications of the composition of the meteors that people the heavens in the months of August and November. The principal result of his observations appears to be, that sodium in a state of luminous vapour is present in the trains left behind these singular bodies. Lightning has also been similarly examined, and lines showing that hydrogen and nitrogen were rendered lu- minous during the electrical discharge, were seen with great distinctness. In fact, the applications of the prism to scientific discovery are almost endless, and in describing them it is difficult to tell where to draw the line. Before quitting this subject, it will be as well to say a few words on the fluorescent rays of the spectrum, to which allusion has already been made towards the end of Chapter IV., Part II. It was there said that the chemical power of the spectrum extends to some dis- tance beyond the extreme violet, a fact that may be readily proved by exposing a piece of photographic paper to the action of the dark portion of the spectrum. Professor Stokes found that there were means of render- ing these rays visible to the eye by altering their rate of vibration. This he found was possible by passing them through the solutions of certain substances, such as sulphate of quinine, horse-chestnut bark, &c. We have already said, that light vibrating at the rate of from 458 to 727 billion times a second, was capable of exciting luminous sensations upon the optic nerve. The latter is the rate of vibration of the extreme violet ray, and it has been found that the eyes of many persons are 262 THE WONDERS OF OPTICS. not sufficiently sensitive to be influenced by it ; it is, therefore, just probable that there are animals whose eyes are so much more sensitive than ours, that they can see rays that exist far beyond those seen by us. Now, as difference of colour is produced by difference in the rate of vibration, it follows that those whose eyes are sensitive enough to perceive the extreme violet rays, see tints of violet that are inappreciable by others. The power of sulphate of quinine in reducing the luminous vibrations is easily seen by passing a tube filled with the solution successively through each of the colours of the spectrum formed by a quartz prism ; the ordinary colours will pass through the liquid as if it were simply water, but on arriving near the violet extremity a gleam of pale blue light will shoot across the tube, and continue to increase. As it is moved onwards the light will gradually die away, until a point is reached nearly equal in length to the whole of the visible spectrum, when it will disappear altogether. It is somewhat singular that no substance has yet been found that will increase the refrangibility of the dark rays beyond the red end of the spectrum. There are many artificial flames which produce this dark light (if we may use such a paradoxical expression) in greater quantity than the sun, whose light is no doubt greatly deteriorated in this respect during its passage through the atmosphere. The substance of which the prism is made also greatly influences the length of the invisible por- tion of the spectrum. By using a quartz prism and lenses of the same material Professor Stokes, found that the spectrum of the electric light could be traced for a distance equal to six times that of the visible portion. The action of certain substances in rendering the in- visible rays of light perceptible may be easily shown by any one possessing a horse-chestnut tree. A weak decoction of the inner portion of the bark having been THE SPECTROSCOPE. 263 made and filtered through blotting-paper, or at any rate allowed to settle, the room is made quite dark and a piece of common brimstone is ignited. The pale blue light given off is comparatively feeble, but it is very rich in the ultra-violet rays ; consequently, when the infusion of horse-chestnut bark is poured into a tall jar of water, beautiful waves of phosphorescent light are seen flashing backwards and forwards as the two liquids mingle. The tincture of stramonium is also possessed of this property, and characters traced on paper with it, although nearly invisible by ordinary daylight, appear distinctly when examined by the light of burning sulphur. 264 THE WONDERS OE OPTICS. CHAPTER X. SPECTRES — THE GHOST ILLUSION* We close our account of the wonders of optics by a description of the ghost illusion, which has been exhi- bited with such great success by M. Robin, the well- known French conjurer, Mr. Pepper, the enterprising manager of the Royal Polytechnic Institution, and seve- ral others. Before doing so, however, we will say a few words on those unpleasant visitations known as spectres, to which some people are liable, either through an over- worked brain or some organic disease. The peculiar appearances known as spectres in optics are certain illusions of vision in which an object is ap- parently presented to the view whi3h does not really exist. In such cases either the brain, the retina, or the optic nerve are unnaturally excited, and made sensitive to an appearance that, physically speaking, does not exist. There is such a close connexion between the senses and the mind, that we continually, and without knowing it, transfer to the physical world that which belongs to the domain of thought. A picture which has struck us during the day will reappear to us at night during sleep, with every detail perfect, or possibly under a form modified by the capricious wanderings of our thoughts. A sudden fright may sometimes be the cause of optical illusions which will pursue us unceasingly. Fear, despair, passion, ambition, and other violent men- SPECTRES — THE GHOST ILLUSION. 265 tal phases, are capable of evoking images closely con- nected with the state of our brain, appearances that we often take for realities, and whose truths we have to test by our faculty of reasoning, before we can set them down as positive illusions. " In the most insignificant phenomena," says Sir David Brewster, "we find that the retina is so powerfully influenced by exterior im- pressions as to retain the images of visible objects for a long time after they have passed out of sight ; besides, this portion of the eye is so strongly influenced by local impressions of which we know neither the nature nor the origin, that we see the shapeless forms of coloured light moving about in the dark. In fact we have, in the cases of Newton and many others, examples of the ease with which the imagination revivifies the images of luminous objects for months or even years, after these impressions took place. After the occurrence of such phenomena, the mind can readily comprehend how thin is the division that separates reality from those spectral illusions which during a particular state of health have afflicted the most intelligent men, not merely those belonging to the community at large, but also the most learned philosophers." Spectres may properly be divided into two classes, those which may be termed subjective, which result from some unnatural action of our minds or bodies, and which properly belong to the science of physiology, and those which may be called objective, which are caused by some peculiar illusion acting on us from without. We shall pass lightly over the first, illustrating them by a single example, while we shall pay more serious attention to those belonging to the second class. Sir Walter Scott, in his Letters on Demonology and Witchcraft, mentions a remarkable instance of the first order of spectres. A doctor of eminence was called in to attend a gentleman who occupied a high place in a 266 THE WONDERS OF OPTICS. particular department connected with the administration of justice. Until the time that the physician's services became necessary, he had shown strong common sense and extraordinary firmness and integrity in every case in which he had been called upon to arbitrate. But after a certain epoch his temper became saddened, although his mind preserved its habitual strength and calmness. At the same time, the feebleness of his pulse, the loss of appetite, and impaired digestion seemed to point out to his medical adviser the existence of some serious source of disturbance. At first the sick man seemed inclined to keep the cause of the change in his health a profound secret; but his melancholy bearing, confused answers, and the badly disguised constraint with which he sharply replied to the interrogations of the doctor, caused the latter to seek for information as to the cause of the disorder in other directions. He made minute inquiries of the various members of his unhappy pa- tient's family, but he could obtain no explanation of the mystery. Every one was lost in conjecture as to the reason of the alarming condition of the patient, which did not appear to be justified by any loss of fortune or beloved friends. His age rendered the idea of an unsuc- cessful love affair improbable, and his known integrity precluded the possibility of remorse. The doctor accord- ingly was compelled to return once more to the straight road, and he used the most serious arguments with his pa- tient to induce him to conquer his obstinacy. At last the doctor's efforts took effect ; the patient allowed him- self to be convinced, and manifested his desire to open his mind frankly to the doctor. They were accordingly left alone, all the doors were securely fastened, and the patient made the following singular avowal. " You cannot be more firmly convinced, my dear friend, than I am myself, that I am on the eve of death, crushed by the fatal malady which has dried up the SPECTRES — THE GHOST ILLUSION. 267 sources of my life. You remember, without doubt, the disease of which the Duke of Olivarez died in Spain ?" "From the idea," replied the doctor, " that he was pursued by an apparition in whose existence he did not believe^ and he died from the continual presence of this imaginary vision weighing down his strength, and break- ing his heart." " Well, my dear doctor," the patient went on, " I am in the same condition, and the presence of the vision that persecutes me is so painful and frightful, that my reason is totally helpless in controlling the effects of my imagination, and I feel that I am dying from the effects of an imaginary illness. My visions began two or three years since. At first I found myself embarrassed from time to time by the presence of a great cat, which ap- peared and disappeared I knew not how. But at last the truth flashed across my mind, and I was compelled to look upon the creature, not as an ordinary domestic animal, but as a vision which had its origin in some de- rangement of the organs of sight or in my imagination. I have no antipathy to cats, in fact I am rather fond of them, so I endured the presence of my imaginary com- panion so well that at last I treated the whole affair with indifference. But at the end of several months the cat disappeared, and was replaced by a spectre of greater importance, and whose exterior was, to say the least of it, very imposing. It was neither more nor less than one of the high officials of the House of Lords, in the full dress belonging to his dignity. " This* personage, who was in court dress, with a bag- wig on his head, and a sword by his side, his coat splen- didly embroidered and his ehajieau bras under his arm, glided along by my side like a shadow. Whether I was in my own house or elsewhere, he mounted the stairs before me, as if to announce my coming. Sometimes he seemed to mix with the company, although it was evident 268 THE WONDERS OF OPTICS. that no one remarked his presence, and I was the sole witness of the chimerical honours that this imaginary individual seemed to render to me. This phantasy of my brain did not make a very strong impression on me, although it made me conceive doubts as to the state of my health, and the effects it would produce upon my reason. " This second phase of my malady, like the first, also came to an end. Some months after, the usher of the Upper House ceased showing himself, and he was re- placed by an apparition that was at once wearing to the mind and terrible to the sight. It was a skeleton. Whether I was alone or in company this frightful image of death never quitted me ; it dogged my footsteps and followed me everywhere, and seemed to be a shadow inseparable from myself. It was in vain that I repeated to myself a hundred times over that the vision was not real, and was only an illusion of my senses. The rea- soning of philosophy and my religious principles, strong though they are, are powerless to triumph over the in- fluence that besets me, and I feel that I shall die a victim to this cruel evil. ,, "It seems then/' interrupted the doctor, " that this skeleton is always before your eyes ?" " It is my evil fate to see it continually before me." "In which case it is at this moment visible to your eyes ?" " It is at present." " And in what part of the room do you imagine that you see it now ?" asked the doctor. " At the foot of my bed," replied the patient : " when the curtains are half open I can see it place itself in the empty space between them." " You say that you are convinced that it is only an illu- sion," replied the doctor ; "have you the firmness to con- vince yourself of it positively ? Have you the necessary Fig. 72. — The Spectre. An optical illusion. SPECTRES — GHOST ILLUSION. 269 courage to get up and go and place yourself in the position which appears to be occupied by the spectre, in order to demonstrate to yourself positively that it is only a vision ?" The unfortunate man sighed and shook his head. "Well," went on the doctor, " let us try another plan." He quitted the chair on which he was sitting, at the head of his patients bed, and placing himself between the half opened curtains, in the place where the patient had pointed out the skeleton, he asked if the apparition was still visible. "Not the whole of it," answered the patient, "be- cause you are standing between him and me; but I see his skull looking at me over your shoulder." In spite of his philosophy, the learned physician could not help starting to hear that the spectre was immedi- ately behind him. He had recourse to other questions, and tried endless remedies, but without success. The prostration of the patient, however, increased, and he died in the same distress of mind in which he had passed the last months of his life. This example is a sad proof of the power of the imagination over the life of the body even when the terrors endured are powerless in destroy- ing the judgment of the unfortunate sufferer. We will say more ; men who have the strongest nerves are not free from similar illusions. The second kind of spectres, in which the science of optics plays so important a part, is the result of the imagination being deceived by art with the assistance of science. These spectres are displayed in the ghost trick which has been practised at various Parisian theatres for a number of years, with very great success, more especially at the Theatres du Chdtelet and Dejazet. The Adelphi, in London, also employed Mr. Pepper to heighten the 270 THE WONDERS OF OPTICS. effect of the excellent actingof Mr. Toole and Mrs. Alfred Mellon, in the dramatic version of Dickens' " Haunted Man," by the introduction of various spectral effects. And the same trick was also called into requisition with some success in several of the minor theatres in New York and other cities of the United States. At the Polytechnic, in London, very remarkable effects were produced, and few who ever saw them will forget the surprise they felt at seeing the first representation of an imponderable ghost endowed with motion, and even speech. Amongst the most successful pro- ductions in this way was the entertainment of M. Robin, one of the cleverest of the many successors of the great Robert Houdin, the prince of prestidigitators. M. Robin claims to be the inventor of the ghost illusion, and to have shown it frequently since 1847. Whether this be so or not it is not our business to decide, but we can testify that his exhibition in the Boulevard du Temple drew all Paris to see it. Evening after evening he not only " called spirits from the vasty deep," but " made them come." He pierced them with swords, he fired pistols through them, and he made them appear and disappear at his slightest wish. He showed the Zouave at Inkermann, lying dead amongst a heap of slain, who at the familiar sound of the drum, rose, pale and grave, and showed the bleeding wounds from which he died. Amongst other scenes shown by M. Robin was one of a spectre appearing to an armed man, who after trying in vain to shut out the vision from his sight fires a pistol at the intruder. Fig. 72 shows the scene as seen by the audience, and fig. 73, the method by which the illusion is worked. The theatre is shown in section. On the left, at the end, are seen the spectators; on the right is the stage upon which the scene is represented. Beneath the stage is an actor clothed in white to personate a ghost, whose image is reflected by the glass above. SPECTRES — GHOST ILLUSION. 275 This glass is placed at an angle, and fills up the whole of the front of the stage, the edges being carefully con- cealed by curtains. The glass of course must be of a very large size, and should be of the very best quality, so that it cannot be seen by the^ audience. The actor must take care to place himself in such a position as to counteract the effect produced by the glass being placed at an angle. At first the cavalier is seen sitting at a table. After soliloquizing for a time in a very remorse- ful manner touching several murders that he has com- mitted, the ghost of one of his victims gradually appears. This is effected by gently turning the electric light upon the concealed actor. The aurderer and victim parley for a short time, when the former, being unable to withstand the reproaches of the ghost any longer, fires a pistol at him point-blank. The ball of course takes no effect, so the villain draws a sword, but before it has left its scabbard the spirit of the victim has vanished with a mocking laugh, or, in other words, the electric light is suddenly turned off. Tb^ iaa/iiigement of the light is exceedingly difficult unaer tnese circumstances ; the theatre, the stage, and the portion beneath ought to be lighted in a very careful manner, for if either is too bright or too dark it mars the whole effect. It must be remembered, too, that the person performing the part of the spectre and the real actor above cannot see each other, consequently all their action has to be carried on by guess-work. The actor below has to walk along an inclined plane, keeping himself exactly at right angles to it. Again, the movements of the latter are obliged to be reversed; for the cavalier already mentioned drew his sword with his left hand in order that the reflected figure should appear to use the right. When well arranged, the ghost trick leaves far behind all the efforts of a similar nature that were obtained by the ancients in the way of magical illusions. It is also 276 THE WONDERS OF OPTICS. incontestably true, contrary to what some people have supposed, that they were unable to perform this illusion in the way we have described, for they were ignorant of the method of manufacturing and polishing glass plates of sufficient size and clearness for the purpose. The production of living but impalpable spectres is thus a completely modern achievement, as we have already proved, and which has taken its place amongst the applications of science to stage art, to the total ex- clusion of all effects depending for their production on the old-fashioned phantasmagoria and magic lantern. THE END. V