The fairy tales of science/Two Eyes are better than One
Two Eyes are better than One.
"Mine eyes are made the fools o' the other senses."
The old proverb which, heads this chapter is suggestive of many wonderful truths connected with vision. Science has demonstrated that two eyes are better than one, for many reasons. We require two eyes to estimate distances, and to obtain a true idea of the roundness, relief, and solidity of natural objects. Those ugly one-eyed fellows who helped Vulcan to forge the thunderbolts, must have been clumsy workmen, in spite of what the ancient writers say to the contrary.
Before we consider the use of two eyes, let us examine the structure of a single organ. The eye has often been compared with the camera obscura, that dark box in which an image is formed of external objects, by means of an arrangement of glass lenses. The eye is, indeed, a dark chamber furnished with lenses, but here the likeness ceases, as its marvellous arrangements are infinitely more beautiful than those of any optical instrument devised by the ingenuity of man.
The human eyeball is a globular mass, somewhat flattened in front, and about the size of a walnut. The white part surrounding the centre is called the sclerotic coat, deriving its name from a Greek word expressive of hardness. This white coat is continued round the back of the eyeball, and forms a sort of strong bag for containing the other parts of the eye. As it is perfectly opaque, it is not continued over the front of the eye, but joins the beautiful transparent membrane called the cornea, or horny coat, which bulges forward a little, and forms that wonderful bow- window through which the rays of light pass to the brain. Within or behind the cornea may be perceived the iris, a sort of coloured fringe which assumes different hues in different eyes, being dark brown, blue, hazel, or grey, and, in exceptional cases, red. When we speak of blue eyes or hazel eyes, we refer to the colour of this remarkable fringe or curtain. In the centre of the eye, surrounded by the iris, is a dark circular space of variable dimensions, called the pupil, which is in fact the opening through which light passes into the dark chamber of the eye.
The internal structure of this wonderful organ is very complicated. The hard white membrane is lined by a coat called the choroid, which is covered on the inside with a perfectly black pigment, and this again with a delicate network of nerves called the retina. The cavity surrounded by these coats is filled by three substances, called humours. Behind the cornea or bow-window is the aqueous humour, a perfectly limpid liquid resembling water; the second in situation is the crystalline humour, which is a little capsule of transparent membrane, holding a small quantity of fluid; and the third, termed the vitreous humour, is a transparent jelly which fills the inner chamber of the eye, and contributes chiefly to preserve the globular figure of the organ.
Each eye is placed in a basin-shaped cavity in the skull, called the orbit, and there are various muscles attached to different parts of the orbit, which by their contraction give a lateral or rolling motion to the eyeball, and thus assist in directing the sight towards particular objects. Eyelids, also moved by muscles, and fringed by the eyelashes, serve to guard the eyes from dust, and to screen them from the access of too intense a light.
So much for the anatomy of the eye; let us now consider its functions. As already observed, the eye may be compared to a camera obscura, for the rays of light from any object entering the pupil form an image on the retina, just as the picture is painted on the ground glass of the camera. The various humours of the eye form a wonderful compound lens, far excelling the achromatic lenses of the opticians. The seat of vision is generally supposed to be the retina, though some philosophers regard the choroid coat as the sensitive tablet upon which the impression is made. We may trace the phenomena of vision up to this point, but no further. We know that a distinct image is formed upon one or other of the delicate coats of the eye, but the manner in which the sensation is conveyed to the brain is an inscrutable mystery. "It is the boast of science," says Herschel, "to have been able to trace so far the refined contrivances of this most admirable organ, not its shame to find something still concealed from scrutiny; for, however anatomists may differ on points of structure, or physiologists dispute on modes of action, there is that in what we do understand of the formation of the eye so similar, and yet so infinitely superior to a product of human ingenuity; such thought, such care, such refinement, such advantage taken of the properties of natural agents, used as mere instruments for accomplishing a given end, as force upon us a conviction of deliberate choice and premeditated design, more strongly, perhaps, than any single contrivance to be found, whether in art or nature, and renders its study an object of the greatest interest."
The Cyclops had each a single eye stuck in the centre of the forehead, but we are provided with a pair of these matchless instruments. Each eye receives an impression of an object, nevertheless we do not see the object double. So long as each image falls exactly on the same part of each sensitive surface, the mind will perceive but one object, and the muscles which move the eyes act in such perfect unison that this result is constantly attained.
If we look at a candle placed at a distance of about ten feet, we see it distinctly as one object, because our eyes are so adjusted that the image of the candle is projected on similar parts of each retina. But if we now hold up a finger about ten inches from the eyes, and look steadily at it, the candle will be seen on both sides of the finger. The eyes are now adjusted to the finger, and the image of the candle no longer falls on the same parts of the two retinæ. Again, if the eyes be directed to the light, the finger will be seen double, because the optic axes are now adjusted to perceive objects at a distance of ten feet. Similar effects may be produced by pressing one eyeball with the finger so as to displace its optical axis, or by getting intoxicated, an experiment which we trust our readers will never perform.
We make use of our two eyes as a pair of compasses to measure distances, for we involuntarily associate the idea of smallness with the convergence of the visual axis, and that of vastness with its divergence. We feel that an object is near or remote, small or large, by opening and shutting our magic compasses, the legs of which are imaginary lines passing through the eyeballs. A person suddenly deprived of one eye estimates the distance of objects with the greatest difficulty; but after some time, experience teaches the one eye to measure distance by the change of focus alone. Let the reader close one eye, and try to snuff a candle, he will then see the import of the old proverb, "Two eyes are better than one."
We have said that two eyes are required in order to form a true conception of solidity; this point we now proceed to consider. If the reader will look at any near object, a book placed on end, for instance, he will at once perceive that it is a real book and not a picture of one; he will see that it has a certain relief; that one portion of it is nearer to him than another; in a word, that it is solid. Now, by closing each eye in turn, the reader will find that one eye will see round one side of the object, and the other round the other side, two different impressions being obtained. Every solid object, therefore, is seen differently by the two eyes, and it has been found that the effect of solidity is produced by the combination of these different impressions in the mind. Two eyes are better than one, not merely because they give symmetry to the face, but because they act together in producing on the inner or mental eye, a perfect and instantaneous impression of the form and position of objects.
This important truth has been revealed by the beautiful and well-known instrument called the stereoscope, which, however, is much better known than understood. Some account of this magic instrument certainly merits a place amongst the fairy tales of science.
The stereoscope, in its most popular form, is simply a small wooden box, furnished with two lenses, like an opera-glass. A double picture, say a photograph of a statue, is placed at the bottom of the box, and viewed with both eyes, by means of the lenses. The effect is truly marvellous, for the design immediately appears in relief the picture becomes a piece of sculpture! This illusion is so perfect, and the means by which it is produced so simple, that we cannot wonder at the popularity which the stereoscope has so rapidly attained.
The term stereoscope is derived from two words in the Greek language, the first signifying a solid body, and the latter vision; it may therefore be freely translated as "that which shows every object in relief." Our readers will admit that the name is a good one, and perfectly descriptive of the powers of the instrument.
Let us now consider how the wonderful illusions of the stereoscope are effected. We shall not require diagrams to make our meaning clear, since every one must be familiar with the construction of the magic instrument.
The two pieces of glass that are placed in the front of the stereoscope are wedge-shaped, that is to say, their outer edges are a little thicker than their inner edges. These glasses act like prisms, and by bending the rays of light that proceed from the double picture, they cause the two halves to combine, and appear as a single picture occupying a central position between the eyes. Two distinct images are thus formed in the eyes, but in consequence of the bending of the rays of light, they are projected upon similar parts of the two retinæ, and seem to be produced by a single object. Whether the two impressions are made by the double picture or by a single solid, the same sensation is produced, as in either case the mind combines the two impressions into the idea of solidity. The stereoscope, therefore, enables us to give a true notion of the form and position of objects from two flat representations on paper or glass; in fact, we may see the objects quite as well as if they stood before us.
Although the stereoscope was discovered some twenty years ago, it has only lately become popular. So long as mere drawings by hand were used as stereoscopic slides, only regular bodies, such as crystals and geometric solids, could be represented; but now, by the aid of photography, we may obtain pictures of any natural or artistic objects.
When we look at a double photograph in the stereoscope, the picture to the right is seen by the right eye only, and that to the left, by the left eye. The two pictures are taken from different points of view, and are exactly similar to the views we obtain of solid objects, by alternately closing the right and left eyes. There is, therefore, no longer any doubt as to the use of two eyes, since by the aid of photography we may obtain pictures similar to those which the eyes receive, and these pictures combine to produce the effect of solidity.
We are indebted to Professor Wheatstone for the discovery of the stereoscope, a discovery which Herschel has truly characterized as "one of the most curious and beautiful for its simplicity in the entire range of experimental optics." The original form of the instrument has been considerably modified by Sir David Brewster, who may indeed be regarded as the inventor of the refracting or popular stereoscope.
We will not attempt to describe the innumerable family groups, landscapes, portraits, and Alpine views, that photography has furnished for the stereoscope. Our readers are doubtless familiar with them, as the stereoscope has become quite a fashionable instrument, and has taken the place of the album upon almost every drawing-room table.
Two eyes are unquestionably better than one, nevertheless persons with but one eye are able to see distinctly. This fact does not refute what we have said about double vision. A person with one eye judges of the relief of an object from the distribution of light and shade, but his perceptions are much less vivid than those of a person with two eyes. It has, moreover, been remarked that a one-eyed person when looking at a solid object is constantly changing the position of the head from side to side, and by this means he obtains with one eye the same result that is obtained by two eyes with the head stationary.
Our readers will now understand why they have two eyes instead of one, and will be able to expound the mysteries of that magic spy-glass, the stereoscope.