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.

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14. The accessory 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. 76, 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. 76, 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 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. 76, 7'),

FIG. 76.-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.

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. 76, 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. 76, 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 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. 76, 5, and 77, 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.

16. This choroid coat (Fig. 76, 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. 75). 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; indeed these cells may perhaps, be more truly considered as part of the retina than as part of the choroid. 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. 77, c, and 76, 5), are the abovementioned ciliary processes.

17. The iris itself (Figs. 76, 7, and 77, a, b) is, as has been already said, a curtain with a round hole in the middle, provided with circular and radiating unstriped muscular fibres, and capable of having its central aperture enlarged or diminished by the action of these fibres, the contraction of which, unlike that of other unstriped muscular fibres, is extremely rapid. The edges of the iris are

firmly connected with the capsule of the eye, at the junction of the cornea and sclerotic, by the connective tissue which enters into the composition of the so-called ciliary ligament. Unstriped muscular fibres, having the same attachment in front, spread backwards on to the outer surface of the choroid, constituting the ciliary muscle (Fig. 76, 4). If these fibres contract, it is obvious that they will pull the choroid forwards; and as the frame, or suspensory ligament of the lens, is connected with the ciliary processes (which simply form the anterior termination of the choroid), this pulling forward of the choroid comes to the same thing as a relaxation of the tension of

FIG. 77.-VIEW OF FRONT HALF OF THE EYEBALL SEEN FROM BEHIND, a, circular fibres; b, radiating fibres of the iris; c, ciliary processes; d, choroid. The crystalline lens has been removed.

that suspensory ligament, which, as I have just said, is in an ordinary condition stretched somewhat tight, keeping the front of the lens flattened.

The iris does not hang down perpendicularly into the space between the front face of the crystalline lens and the posterior surface of the cornea, which is filled by the aqueous humour, but applies itself very closely to the anterior face of the lens, so that hardly any interval is left between the two (Figs. 76 and 78).

The retina, as we have seen, lines the interior of the eye, being placed between the choroid and vitreous humour,

its rods and cones being imbedded in the pigment epithelium lining the former, and its inner limiting membrane touching the latter.

About a third of the distance back from the front of the eye the retina seems to end in a wavy border called the ora serrata (Fig. 76, 9), and in reality the nervous elements of the retina do end here, having become considerably reduced before this line is reached. Some of the connective tissue elements however pass on as a delicate kind of membrane at the back of the ciliary processes towards the crystalline lens.

18. The eyeball, the most important constituents of which have now been described, is, in principle, a camera of the kind described above-a water camera. That is to say, the sclerotic answers to the box, the cornea to the watch-glass, the aqueous and vitreous humours to the water filling the box, the crystalline to the glass lens, the introduction of which was imagined. The back of the box corresponds with the retina.

But further, in an ordinary camera obscura, it is found desirable to have what is termed a diaphragm (that is, an opaque plate with a hole in its centre) in the path of the rays, for the purpose of moderating the light and cutting off the marginal rays which, owing to certain optical properties of spheroidal surfaces, give rise to defects in the image formed at the focus.

In the eye, the place of this diaphragm is taken by the iris, which has the peculiar advantage of being self-regulating dilating its aperture, and admitting more light when the light is weak; but contracting its aperture and admitting less light when the illumination is strong.

19. In the water camera, constructed according to the description given above, there is the defect that no provision exists for adjusting the focus to the varying distances of objects. If the box were so made that its back, on which the image is supposed to be thrown, received distinct images of very distant objects, all near ones would be indistinct. And if, on the other hand, it were fitted to receive the image of near objects, at a given distance, those of either still nearer, or more distant, bodies would be blurred and indistinct. In the ordinary camera this