The Wonders of Optics — Story, Setting & Ideas

THE STRUCTURE OF THE EYE 22

THE ERRORS OF THE EYE 30

THE APPRECIATION OF COLOUR 44

ILLUSIONS CAUSED BY LIGHT ITSELF 53

THE INFLUENCE OF THE IMAGINATION 60

THE SOLAR SPECTRUM 84

OTHER CAUSES OF COLOUR 94

LUMINOUS, CALORIFIC, CHEMICAL, AND 100 MAGNETIC PROPERTIES OF THE SPECTRUM

THE LAWS OF REFLECTION.—MIRRORS 106

METALLIC BURNING MIRRORS 117

OPTICAL INSTRUMENTS.—THE SIMPLE AND 141 COMPOUND MICROSCOPE. THE SOLAR AND PHOTO-ELECTRIC MICROSCOPE

THE TELESCOPES OF GALILEO, GREGORY, 150 NEWTON, HERSCHEL, LORD ROSSE, AND FOUCAULT

THE MAGIC LANTERN 173

THE PHANTASMAGORIA 183

OTHER OPTICAL ILLUSIONS 196

THE PROPERTIES OF MIRRORS 216

POLYORAMA—DISSOLVING VIEWS—DIORAMA 231

THE CAMERA OBSCURA AND CAMERA LUCIDA 242

SPECTRES—THE GHOST ILLUSION 264

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LIST OF ILLUSTRATIONS.

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 Rays _ib._

8. The Recomposition of Light 86

9. Recomposition of Light by means of a 87 Concave Mirror

10. Recomposition of Light by means of a 88 number of Mirrors

12. Newton’s Rings 95

13. Reflection from Plane Surfaces 107

15. Experimental Proof of Refraction _ib._

16. The Effects of Plane Mirrors 109

17. Reflection from the Surface of Water 110

18. Concave Mirror 111

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 _ib._ Lens

30. Conjugate Foci 130

31. Images formed by Convex Lenses 131

32. Magnifying Property of Convex Lenses 132

33. Diminishing Effect of Concave Lenses _ib._

34. Cannon of the Palais Royal 134

35. Fresnel’s 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 of a Newtonian Telescope 162

47. Herschellian Telescope 164

48. Foucault’s Large Telescope 169

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

56. Nostradamus and Marie de Médicis 201

57. The Arrangement of the Reversing Prism 203

58. The Goat Trick 205

59. How to see through a Brick 207

60. The Polemoscope 210

61. Protection against ill-natured People 213

64. Effect of Cut Paper-work 225

65. Seditious Toys 229

69. The Principle of the Refracting 239 Stereoscope

70. The Camera Obscura 243

71. Section of Camera Lucida 247

72. The Spectre—an Optical Illusion 269

73. How to produce Spectres 271

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THE WONDERS OF OPTICS.

THE PHENOMENA OF VISION.

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

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

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

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

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

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

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

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

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

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

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

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

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CHAPTER II. THE STRUCTURE OF THE EYE.

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

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

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

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

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

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

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

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

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

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