of the retina in this part, appears in the field of view. If the bright light be of one colour, the part of the retina on which it falls becomes insensible to rays of that colour, but not to the other rays of the spectrum. This is the explanation of the appearance of what are called complementary colours. For example, if a bright red wafer be stuck upon a sheet of white paper, and steadily looked at for some time with one eye, when the eye is turned aside. to the white paper a greenish spot will appear, of about the size and shape of the wafer. The red image has, in fact, fatigued the part of the retina on which it fell for red light, but has left it sensitive to the remaining coloured rays of which white light is composed. But we know that if from the variously coloured rays which make up the spectrum of white light we take away all the red rays, the remaining rays together make up a sort of green. So that, when white light falls upon this part, the red rays in the white light having no effect, the result of the operation of the others is a greenish hue. If the wafer be green, the complementary image, as it is called, is red.

8. In some persons, the retina appears to be affected in one and the same way by rays of light of various colours, or even of all colours. Such colour-blind persons are unable to distinguish between the leaves of a cherry-tree and its fruit by the colour of the two, and see no difference between blue and yellow cloth.

This peculiarity is simply unfortunate for most people, but it may be dangerous if unknowingly possessed by railway guards or sailors. It probably arises either from a defect in the retina, which renders that organ unable to respond to different kinds of luminous vibrations, and consequently insensible to red rays or yellow rays, &c., as the case may be, or it may proceed from some unusual absorptive power of the humours of the eye which prevents particular rays from reaching the retina; or the fault may lie in the brain itself.

9. The sensation of light may be excited by other causes than the impact of the vibrations of the luminiferous ether upon the retina, Thus, an electric shock sent through the eye, gives rise to the appearance of a flash of light and pressure on any part of the retina produces a luminous image, which lasts as long as the

pressure, and is called a phosphene. If the point of the finger be pressed upon the outer side of the ball of the eye, the eyes being shut, a luminous image-which, in my own case, is dark in the centre, with a bright ring at the circumference (or, as Newton described it, like the "eye" in a peacock's tail)-is seen; and this image lasts as long as the pressure is continued. Most persons, again, have experienced the remarkable display of subjective fireworks which follows a heavy blow upon the eyes, produced by a fall from a horse, or by other methods well known to English youth.

It is doubtful, however, whether these effects of pressure, or shock, really arise from the excitation of the retina proper, or whether they are not rather the result of the violence done to the fibres of the optic nerve apart from the retina.

10. The last paragraph raises a distinction between the "fibres of the optic nerve" and the "retina" which may not have been anticipated, but which is of much importance.

Conse

We have seen that the fibres of the optic nerve ramify in the inner or anterior fourth of the thickness of the retina, while the layer of rods and cones forms its outer or posterior fourth. The light, therefore, must fall first upon the fibres of the optic nerve, and, only after traversing them, can it reach the rods and cones. quently, if the fibrillæ of the optic nerve themselves are capable of being affected by light, the rods and cones can only be some sort of supplementary optical apparatus. But, in fact, it is the rods and cones which are affected by light, while the fibres of the optic nerve are themselves insensible to it. The evidence on which this statement rests is

a. The blind spot is full of nervous fibres, but has no cones or rods.

b. The yellow spot, where the most acute vision is situated, is full of close-set cones, but has no nerve fibres.

c. If you go into a dark room with a single small bright candle, and, looking towards a dark wall, move the light up and down, close to the outer side of one eye, so as to allow the light to fall very obliquely into the eye,

one of what are called Purkinje's figures is seen. This is a vision of a series of diverging, branched, red lines on a dark field, and in the interspace of two of these lines is a sort of cup-shaped disk. The red lines are the retinal blood-vessels, and the disk is the yellow spot. As the candle is moved up and down, the red lines shift their position, as shadows do when the light which throws them changes its place.

Now, as the light falls on the inner face of the retina, and the images of the vessels to which it gives rise shift their position as it moves, whatever perceives these images must needs lie on the other, or outer, side of the vessels. But the fibres of the optic nerve lie among the vessels, and the only retinal structures which lie outside them are the granular layers and the rods and cones.

d. Just as, in the skin, there is a limit of distance within which two points give only one impression, so there is a minimum distance by which two points of light falling on the retina must be separated in order to appear as two. And this distance corresponds pretty well with the diameter of the cones.

11. The impact of the ethereal vibrations upon the sensory expansion, or essential part of the visual apparatus alone, is sufficient to give rise to all those feelings, which we term sensations of light and of colour, and to that feeling of outness which accompanies all visual sensation. But, if the retina had a simple transparent covering, the vibrations radiating from any number of distinct points in the external world would affect all parts of it equally, and therefore the feeling aroused would be that of a generally diffused luminosity. There would be no separate feeling of light for each separate radiating point, and hence no correspondence between the visual sensations and the radiating points which aroused them.

It is obvious that, in order to produce this correspondence, or, in other words, to have distinct vision, the essential condition is, that distinct luminous points in the external world shall be represented by distinct feelings of light. And since, in order to produce these distinct feelings, vibrations must impinge on separate rods or cones, it follows that, for the production of distinct vision, some apparatus must be interposed between the retina and the external world, by the action of which, distinct luminous

points in the latter shall be represented by corresponding points of light on the retina.

In the eye of man and of the higher animals, this accessory apparatus of vision is represented by structures which, taken together, act as a biconvex lens, composed of substances which have a much greater refractive power than the air by which the eye is surrounded; and which throw upon the retina luminous points, which correspond in number, and, in one sense, in position, with those luminous points in the external world from which ethereal vibrations proceed towards the eye. The luminous points thus thrown upon the retina form a picture of the external world—a picture being nothing but lights and shadows, or colours, arranged in such a way as to correspond with the disposition of the luminous or coloured parts of the object represented. 12. That a biconvex lens is competent to produce a picture of the external world on a properly arranged screen is a fact of which everyone can assure himself by simple experiments. An ordinary spectacle glass is a transparent body denser than the air, and convex on both sides. If this lens be held at a certain distance from a screen or wall in a dark room, and a lighted candle be placed on the opposite side of it, it will be easy to adjust the distances of candle, lens, and wall, so that an image of the flame of the candle, upside down, shall be thrown upon the wall.

The spot on which the image is formed is called a focus. If the candle be now brought nearer to the lens, the image on the wall will enlarge, and grow blurred and dim, but may be restored to brightness and definition by moving the lens further from the wall. But if, when the new adjustment has taken place, the candle be moved away from the lens, the image will again become confused, and, to restore its clearness, the lens will have to be brought nearer the wall.

Thus a convex lens forms a distinct picture of luminous objects, but only at the focus on the side of the lens opposite to the object; and that focus is nearer when the object is distant, and further off when it is near.

13. Suppose, however, that, leaving the candle unmoved, a lens with more convex surfaces is substituted for the first, the image will be blurred, and the lens will have to be moved nearer the wall to give it definition. If, on the other hand, a lens with less convex surfaces is sub

stituted for the first, it must be moved further from the wall to attain the same end.

In other words, other things being alike, the more convex the lens the nearer its focus; the less convex, the further off its focus.

If the lens were elastic, pulling it at the circumference would render it flatter, and thereby lengthen its focus; while, when let go again, it would become more convex, and of shorter focus.

Any material more refractive than the medium in which it is placed, if it have a convex surface, causes the rays of light which pass through the less refractive medium to that surface to converge towards a focus. If a watch-glass be fitted into one side of a box, and the box be then filled with water, a candle may be placed at such a distance outside the watch-glass that an image of its flame shall fall on the opposite wall of the box. If, under these circumstances, a doubly convex lens of glass were introduced into the water in the path of the rays, it would act (though less powerfully than if it were in air) in bringing the rays more quickly to a focus, because glass refracts light more strongly than water does.

A camera obscura is a box, into one side of which a lens is fitted, so as to be able to slide backwards and forwards, and thus throw on the screen at the back of the box distinct images of bodies at various distances off. Hence the arrangement just described might be termed a water

camera.

14. The intermediate organs, by means of which the physical agent of vision, light, is enabled to act upon the expansion of the optic nerve, comprise three kinds of apparatus: (a) a water camera," the eyeball; (b) muscles for moving the eyeball; (c) organs for protecting the eyeball, viz. the eyelids, with their lashes, glands, and muscles; the conjunctiva; and the lachrymal gland and its ducts.

The eyeball is composed, in the first place, of a tough, firm, spheroidal case consisting of fibrous or connective tissue, the greater part of which is white and opaque, and is called the sclerotic (Fig. 75, 2). In front, however, this fibrous capsule of the eye, though it does not change its essential character, becomes transparent, and receives

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