Discoveries and Inventions of the Nineteenth Century

gems, and “in gloss of satin and glimmer of pearls.” The eye is also the organ which conveys to our minds the impressions of visible motion, with all those pleasures of exciting spectacle which enter so largely into our enjoyment of life. It likewise discriminates the forms, sizes, and distances of objects; but by a process long misunderstood, and dependent upon a set of perceptions which, although precisely those whence we derive our most fundamental notions of the objects around us, have been completely overlooked in that time-honoured enumeration of the senses which recognizes only five. If such be the extent to which our minds are dependent upon the wonderful apparatus of the eye, it may easily be imagined what must be the comparative narrowness of mental development in those who have never enjoyed this precious sense, and the feeling of deprivation in those, who, having enjoyed it, have unfortunately lost it. Well may our sublime poet despairingly ask— “Since light so necessary is to life, And almost life itself—if it be true That light is in the soul— The all in every part: why was the sight To such a tender ball as the eye confined, So obvious and so easy to be quenched?” —for he himself, in his own person, experienced this deprivation, and he thus touchingly, in his great work, laments his loss: “Thus with the year Seasons return; but not to me returns Day, or the sweet approach of even or morn, Or sight of vernal bloom, or summer’s rose, Or flocks, or herds, or human face divine; But cloud instead, and ever-during dark Surround me; from the cheerful ways of men Cut off; and for the book of knowledge fair Presented with a universal blank Of Nature’s works—to me expunged and rased, And wisdom at one entrance quite shut out.” An organ which is the instrument of so many nice discriminations as is the eye must, of course, present the most delicate adjustment in its parts. So much has in recent times been learnt of the nature of its mechanism; of the relation between the impressions made upon it and the judgments formed by the mind therefrom; of the illusions which its very structure produces; of the defects to which it is liable; and of its wonderfully 454refined physiological elements—that a branch of science sufficiently extensive to require a large part of a studious lifetime for its complete mastery has grown up under the hands of modern physiologists, physicists, and psychologists. To some of the results of their labour we would invite the reader’s attention; and in order to render the account of them intelligible, we must, to a certain extent, describe “things new and old.” THE EYE. Fig. 234.—Vertical Section of the Eye. The form of the human eye and the general arrangement of its parts may be understood by referring to Fig. 234, which is a section of the eyeball. It has a form nearly globular, and is covered on the outside by a tough firm case, A, named the sclerotic coat, which is, for the most part, white and opaque. This covering it is which forms what is commonly termed the “white of the eye;” but in the front part of the eyeball it loses its 455opacity, and merges into a transparent substance, termed the cornea, B. The cornea has a greater convexity than the rest of the exterior of the eyeball, so that it causes the front part of the eye to have a somewhat greater projection than would result from its general globular form. This sclerotic coat—with its continuation, the cornea—serves to support and protect the more delicate parts within, and is itself kept in shape by the humours, which fill the whole of the interior. The greater space is occupied by the vitreous humour, C; but the space immediately behind the transparent cornea is filled with the aqueous humour, D. The latter is little else than pure water, and the former is like thin transparent jelly. The cavities containing these two humours are separated by the transparent double convex lens, E, called the crystalline lens, which, in consistence, resembles very thick jelly or soft gristle. The outward surface of this lens has a flatter curvature than the inner surface. Immediately in front of the crystalline lens is found the iris, F, which may be described as a curtain having in the middle a round hole. The iris is the part which varies in colour from one individual to another—being blue, brown, grey, &c.; and the aperture in its centre is the dark circular spot termed the pupil. The general disposition of the parts of the eye with regard to light will be most easily understood by comparing it with an optical instrument, to which it bears no little resemblance, namely, the camera obscura, so well known in connection with photography. We may picture to ourselves a still more complete resemblance, by imagining that the lens of the camera is single, that we have fixed in front of it a watch-glass, with the convex side outwards, and that we have filled with water the whole of the interior of the camera, including the space between the watch-glass and the lens. The focussing-screen of the camera corresponds with the inner surface of the back of the eyeball, about which we shall presently have more to say. Now, even if the camera had no lens, but were simply a box filled with water, and having in front the watch-glass, fixed in the manner just mentioned, we could obtain the images of objects on the screen, as a consequence of the curvature of the watch-glass. It would, however, in this case, be necessary to have the camera much longer, or, in other words, the rays would be brought to a focus at a greater distance than if we put in the glass lens, which would, thus placed in the water, cause the rays to converge to a focus at a much shorter distance, although its effect when surrounded by water would be less powerful than in the air. There we see the effect of the crystalline lens of the eye in bringing the rays to a focus within a much shorter distance than that which would be required had there been present only the curved cornea, and the aqueous and vitreous humours of the eye, which are but little different from pure water in their optical properties. If we focus the camera by adjusting the distance between the lens and the screen so as to get a distinct image of a near object, we should find, on directing the instrument to a distant one, that the image would be blurred and indistinct, and the lens would have to be moved nearer to the screen; or we could get the image of the distant object distinct by replacing the lens by another lens in the same position, but having some flatter curvature. It is plain that the same object would be gained if our lens could be made of some elastic material, which, on being pulled out radially at its edges, could be made to assume the required degree of flatness without losing its lenticular form. Now, it is precisely with an automatic adjustment of this kind that the crystalline lens of the eye is provided, for the lens is suspended by an elastic ligament, G, by the tension of which its surfaces 456are more flattened than they would otherwise be; but when the tension of this ligament is relaxed, by the action of certain delicate muscles which draw it down, the elasticity of the lens causes it to assume a more convex form. Fig. 235. Section of Retina. These optical adjustments give, on the inner surface of the coats of the eye, a more or less perfect real image of the objects to which the eye is directed, and it is on the back part of this inner surface that the network of nerves, called the retina, H, is spread out. The sclerotic coat, already spoken of, is lined internally with another, named the choroid, which is composed of delicate blood-vessels, intermingled with a tissue of cells filled with a substance of an intensely black colour. It is upon this last layer that the delicate membrane of the retina is spread out between the choroid and the vitreous humour. The retina is, in part, an expansion of the fibres of the optic nerve over the back part of the eyeball. If we suppose the globe of this cut vertically into two portions, and so divide the front from the back part of the eye, the retina would be seen spread out on the concave surface of the back part, and in the middle of this part, opposite the crystalline lens, would be seen a spot in which the retina assumes a yellowish colour, and in the centre of this, a little round pit or depression. The spot is called the macula lutea, or yellow spot, and the little central pit, which is of the highest importance in vision, is termed the fovea centralis. A little way from the yellow spot, and nearer the nose, is a point from which a number of fibres are seen to radiate, and this is, in fact, the part at which the optic nerve enters the eyeball, and from which it sends out its ramifications over the retina. This part, for a reason which will shortly appear, is called the blind spot. When the minute structure of the retina is examined by the microscope, its physiological elements are found to undergo very remarkable modifications at the yellow spot. In the retina, although the total thickness does not exceed the 180th part of an inch, no fewer than eight or ten different essential or nervous layers have been distinguished. Fig. 235 rudely represents a section. The lowest stratum, A, which is next the choroid, and forms about a quarter of the total thickness, is formed of a multitude of little rod-shaped bodies, a, ranged side by side, and among these are the conical or bottle-shaped bodies, b. This lowest stratum of the retina is called the layer of rods and cones. At their front extremities the rods and cones pass into very delicate fibres, which, going through an extremely fine layer of fibres, B, are connected with a series of small rounded bodies, which form 457the layer of nuclei, C, separated by a layer of nervous fibres, D, from a granular layer, E, in front of which is a stratum of still finer granules, F, underlying a layer of ganglionic nerve-cells, G, of a larger size than any of the other elements, and these ganglionic cells send out numerous branching nerve-fibres, forming the layer H. Finally, on the front surface of the retina there is a thin stratum formed of fibres, which issue from the optic nerve, K, Fig. 234, and in fact constitute the expansion of this nerve on the inner surface of the eyeball. The terminations of some, at least, of these nerve-fibres have been traced, and have been found to form junctions with those branching from the ganglionic cells. Fig. 236. Of the part played by each of these delicate structures in exciting visual impressions little is yet known. How light, or the pulsations of ether, if such there be, is ultimately converted into sensation will probably for ever remain a mystery, although it is quite likely that the kind of visual impression which is conveyed by each part of the elaborate structure of the retina may ultimately be distinguished. One curious result of modern investigation is that light falling directly upon fibres of the optic nerve is quite incapable of exciting any sensation whatever. Light has no more effect on this nerve and its fibres than it would have on any other nerve of the body if exposed to its action. The apparatus of rods, cones, and other structures are absolutely essential to enable light to give that stimulus