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CHAPTER VI.

THE SENSE OF SIGHT.

We

It might at first sight seem easy enough to answer the question whether an animal can see or not. In reality, however, the problem is by no means so simple. find, in fact, every gradation from the mere power of distinguishing a difference between light and darkness up to the perception of form and colour which we ourselves enjoy.

The undifferentiated tissues of the lower animals, and even of plants, are, as we all know, affected in a marked manner by the action of light.

But to see, in the sense of perceiving the forms of objects, an animal must possess some apparatus by means of which-firstly, the light coming from different points, a, b, c, d, e, etc., is caused to act on separate parts of the retina in the same relative positions; and secondly, by means of which these points of the retina can be protected from the light coming in other directions.

There are three modes in which it is theoretically possible that this might be effected.

Firstly, let SS" be an opaque screen, with a small orifice at o. Let a b c d e be a body in front of the

THREE POSSIBLE MODES OF SIGHT.

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screen. In this case the rays from the point c can pass straight through the orifice o, and fall on the retina of an eye, or on a flat surface at c'. There is no other direction in which the rays from c could pass through o. In the same way, the light from a would fall on the point a', that from b on b', from don d', and e on e'.

The results which would be given in this way would be, however, very imperfect, and, as a matter of fact, no eye constructed on this system is known to exist.

α

23

d

S

Fig. 74.

Secondly, let a number of transparent tubes or cones with opaque walls be ranged side by side in front of the retina, and separated from one another by black pigment. In this case the only light which can reach the optic nerve will be that which falls on any given tube in the direction of its axis.

For instance, in Fig. 75 the light from a will pass to a', that from b to b', that from c to c', and so on. The light from c, which falls on the other tubes, will not

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DIFFERENT FORMS OF EYE.

reach the nerve, but will impinge on the sides and be absorbed by the pigment. Thus, though the light from e will illuminate the whole surface of the eye, will only affect the nerve at c'.

it

In this mode of vision, which was first clearly explained by Johannes Müller, the distinctness of the image will be greater in proportion to the number of separate cones. "An image," he says,' * "formed by several thousand separate points, of which each corresponds to a distinct field of vision in the external world, will resemble a piece of mosaic work, and a better idea cannot be conceived of the image of external objects which will be depicted on the retina of beings endowed with such organs of vision, than by comparing it with perfect work of that kind."

There is, it will presently be seen, reason to suppose that the compound eyes of insects, crustacea, and some molluscs, are constructed on this plan.

Thirdly, let L (Fig. 76) be a lens of such a form

a

Fig. 76.

that all the light which falls upon its surface from the point a is re-collected at the point a', that from b at b', from c at c', and so on. If now other light be excluded,

* 66 "Phys. of the Senses," by Johannes Müller, translated by Dr. Baly.

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an image of a be will be thrown on a screen or on a retina at a' b'c'. The image, it will be observed, is necessarily reversed. This is the form of eye which we possess ourselves: it is, in fact, a camera obscura. It is that of all the higher animals, of most molluscs, the ocelli of insects, etc.

Fig. 77, taken from Helmholtz, will give an idea of the manner in which we see.

Fig. 77.-G, Vitreous humor; L, lens; W, aqueous humor; c, ciliary process; d, optic nerve; e e, suspensory ligament; kk, hyaloid membrane; ff, hh, cornea; gg, choroid; i, retina; 77, ciliary muscle; mf, nf, sclerotic coat; pp, iris; s, the yellow spot.

The eyeball is surrounded by a dense fibrous membrane, the sclerotic coat, or white of the eye, mf, nf, which

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STRUCTURE OF THE EYE.

passes in front into the glassy, transparent cornea, ƒ ƒ, hh; the greater part of the centre of the eye is occupied by a clear gelatinous mass, the vitreous humor, G, in front of which is the lens, L; while between the lens and the cornea is the aqueous humor, W. The sclerotic coat is lined at the back of the eye by a delicate, vascular, and pigmented membrane—the choroid, g g, so called from the great number of blood-vessels which it contains; in front this membrane joins the iris, p p, which leaves a central opening, the pupil, so called from the little image of ourselves, which we see reflected from an eye when we look into it. The iris gives its colour to the eye, its posterior membrane containing pigment-cells; if these are few in number, it appears blue, from the layer behind shining through, and the greater the number of these cells the deeper the colour. e e, is a peculiar membrane, which serves to retain the lens in its place. The optic nerve, d, enters at the back of the eye, and, spreading out on all sides, forms the retina, i, of which one spot, s, the yellow spot, is pre-eminently sensitive. The action of the eye resembles that of a camera obscura, and, as shown in Fig. 76, the rays which fall upon it are refracted so as to form a reversed picture on the back of the eye.

The retina (Fig. 78) is very complicated, and, though no thicker than a sheet of thin paper, consists of no less than nine separate layers, the innermost (Figs. 78, 79) being the rods and cones, which are the immediate recipients of the undulations of light. Fig. 79 represents the rods and cones isolated and somewhat more enlarged.

The number of rods and cones in the human eye is enormous. At a moderate computation the cones may