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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 snift 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.
II. 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 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 clastic, 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 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
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
the name of the cornea (Fig. 75, 1). The corneal portion of the case of the eyeball is more convex than the sclerotic portion, so that the whole form of the ball is such as would be produced by cutting off a segment from the
FIG. 75. HORIZONTAL SECTION OF THE EYEball. 1, cornea; 1', conjunctiva; 2, sclerotic; 2', sheath of optic nerve; 3, choroid; 3", rods and cones of the retina; 4, ciliary muscle; 4', circular portion of ciliary muscle; 5, ciliary process; 6, posterior chamber between 7, the iris and the suspensory ligament; 7', anterior chamber; 8, artery of retina in the centre of the optic nerve; 8', centre of blind spot; 8", macula lutea; 9, ora serrata (this is of course not seen in a section such as this, but is introduced to show its position); 10, space behind the suspensory ligament (canal of Petit); 12, crystalline lens ; 13, vitreous humour; 14 marks the position of the ciliary ligament; a, optic axis (in the actual eye of which this is an exact copy, the yellow spot happened, curiously enough, not to be in the optic axis); b, line of equator of the eyeball.
front of a spheroid of the diameter of the sclerotic, and replacing this by a segment cut from a smaller, and consequently more convex, spheroid.
15. The corneo-sclerotic case of the eye is kept in shape by what are termed the humours-watery or semi-fluid substances, one of which, the aqueous humour (Fig. 75, 7'), which is hardly more than water holding a few organic and saline substances in solution, distends the corneal chamber of the eye, while the other, the vitreous (Fig. 75, 13), which is rather a delicate jelly than a regular fluid, keeps the sclerotic chamber full.
The two humours are separated by the very beautiful, transparent, doubly-convex crystalline lens (Fig. 75, 12), denser, and capable of refracting light more strongly than either of the humours. The crystalline lens is composed of fibres having a somewhat complex arrangement, and is highly elastic. It is more convex behind than in front, and it is kept in place by a delicate, but at the same time strong and elastic, membranous frame or suspensory ligament, which extends from the edges of the lens to what are termed the ciliary processes of the choroid coat (Figs. 75, 5, and 76, c). In the ordinary condition of the eye this ligament is kept tense, i.e. is stretched pretty tight, and the front part of the lens is consequently flattened against it.
16. This choroid coat (Fig. 75, 3) is a highly vascular membrane, in close contact with the sclerotic externally, and lined, internally, by a layer of small polygonal bodies containing much pigmentary matter, called pigment cells (Fig. 74). These pigment cells are separated from the vitreous humour by the retina only. The rods and cones of the latter are in immediate contact with them. The choroid lines every part of the sclerotic, except just where the optic nerve enters it at a point below, and to the inner side of the centre of the back of the eye; but when it reaches the front part of the sclerotic, its inner surface becomes raised up into a number of longitudinal ridges, with intervening depressions, like the crimped frills of a lady's dress, terminating within and in front by rounded ends, but passing, externally, into the iris. These ridges, which when viewed from behind seem to radiate on all sides from the lens (Figs. 76, c, and 75, 5), are the abovementioned ciliary processes.
17. The iris itself (Figs. 75, 7, and 76, a, b) is, as has been already said, a curtain with a round hole in the