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. 75 and 77).

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 former, and its anterior 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. 75, 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 nearer, or more distant, bodies would be blurred and indistinct. In the ordinary camera this difficulty is overcome by sliding the lenses in and out, a process which is not compatible with the construction of our water camera. But there is clearly one way among many, in which this adjustment might be effected-namely, by changing the glass lens; putting in a less convex one when more distant objects had to be pictured, and a more convex one when the images of nearer objects were to be thrown upon the back of the box.

But it would come to the same thing, and be much more convenient, if, without changing the lens, cne and the same lens could be made to alter its convexity. This is what actually is done in the adjustment of the eye to distances.

20. The simplest way of experimenting on the adjustment of the eye is to stick two stout needles upright into a straight piece of wood, not exactly, but nearly in the same straight line, so that, on applying the eye to one end of the piece of wood, one needle (a) shall be seen about six inches off, and the other (b) just on one side of it at twelve inches' distance.

If the observer look at the needle b, he will find that he sees it very distinctly, and without the least sense of effort; but the image of a is blurred and more or less double. Now let him try to make this blurred image of the needle a distinct. He will find he can do so readily enough, but that the act is accompanied by a sense of effort somewhere in the eye. And in proportion as a becomes distinct, b will become blurred. Nor will any effort enable him to see a and b distinctly at the same time.

21. Multitudes of explanations have been given of this remarkable power of adjustment, but it is only within the last few years that the problem has been solved, by the accurate determination of the nature of the changes in the eye which accompany the act. When the flame of a taper is held near, and a little on one side of, a person's eye, anyone looking into the eye from a proper point of view, will see three images of the flame, two upright and

one inverted. One upright figure is reflected from the front of the cornea, which acts as a convex mirror. The second proceeds from the front of the crystalline lens, which has the same effect; while the inverted image proceeds from the posterior face of the lens, which, being convex backwards, is, of course, concave forwards, and acts as a concave mirror.

Suppose the eye to be steadily fixed on a distant object, and then adjusted to a near one in the same line of vision, the position of the eyeball remaining unchanged. Then the upright image reflected from the surface of the cornea, and the inverted image from the back of the lens, will remain unchanged, though it is demonstrable that their size or apparent position must change if (ither the cornea, or the back of the lens, alter either their form or

Illustrates the change in the form of the lens when adjusted to distant, B to near objects.

their position. But the second upright image, that reflected by the front face of the lens, does change both its size and its position; it comes forward and grows smaller, proving that the front face of the lens has become more convex. The change of form of the lens is, in fact, that represented in Fig. 77.

These may be regarded as the facts of adjustment with which all explanations of that process must accord. They at once exclude the hypotheses (1) that adjustment is the result of the compression of the ball of the eye by its muscles, which would cause a change in the form of the cornea; (2) that adjustment results from a shifting of the lens bodily, for its hinder face does not move; (3) that it results from the pressure of the iris upon the front face

of the lens, for under these circumstances the hinder face of the lens would not remain stationary. This last hypothesis is further negatived by the fact that adjustment takes place equally well when the iris is absent.

One other explanation remains, which is, in all probability, the true one, though not altogether devoid of difficulties. The lens, which is very elastic, is kept habitually in a state of tension by the elasticity of its suspensory ligament, and consequently has a flatter form than it would take if left to itself. If the ciliary muscle contracts, it must, as has been seen, relax that ligament, and thereby diminish its elastic tension upon the lens. The lens, consequently, will become more convex, returning to its former shape when the ciliary muscle ceases to contract, and allows the choroid to return to its ordinary place.

If this be the true explanation of adjustment, the sense of effort we feel must arise from the contraction of the ciliary muscle.

22. Adjustment can take place only within a certain range, which admits of great individual variations. As a rule, no object which is brought within less than about ten inches of the eye can be seen distinctly without effort.

But many persons are born with the surface of the cornea more convex than usual, or with the refractive power of the eye increased in some other way; while, very generally, as age draws on, the cornea flattens. In the former case, objects at ordinary distances are seen indistinctly, because these images fall not on the retina, but in front of it; while, in the latter, the same indistinctness is the result of the rays of light striking upon the retina before they have been brought to a focus. The defect of the former, or short-sighted people, is amended by wearing concave glasses, which cause the rays to diverge; of the latter, or long-sighted people, by wearing convex glasses, which make the rays converge.

In the water camera the image brought to a focus on the screen at the back is inverted; the image of a tree for instance is seen with the roots upwards and the leaves and branches hanging downwards. The right of the image also corresponds with the left of the object and

vice versa. Exactly the same thing takes place in the eye with the image focussed on the retina. It too is inverted. (See Lesson X. § II.)

23. The muscles which move the eyeball are altogether six in number-four straight muscles, or recti, and two oblique muscles, the obliqui (Fig. 78). The straight muscles are attached to the back of the orbit, round the edges of the hole through which the optic nerve passes, and run straight forward to their insertions into the scleroticone, the superior rectus, in the middle line above; one,

A, the muscles of the right eyeball viewed from above, and B of the left eyeball viewed from the outer side; S.R. the superior rectus; Inf. R. the inferior rectus; E.R., In. R. the external rectus; S. Ob. the superior oblique; Inf.Ob. the inferior obliqu; Ch. the chiasma of the optic nerves (II.); III. the third nerve which supplies all the muscles except the superior oblique and the external rectus.

the inferior, opposite it below; and one half-way on each side, the external and internal recti. The eyeball is completely imbedded in fat behind and laterally; and these muscles turn it as on a cushion; the superior rectus inclining the axis of the eye upwards, the inferior downwards, the external outwards, the internal inwards.

The two oblique muscles are both attached on the outer side of the ball, and rather behind its centre; and they both pull in a direction from the point of attachment towards the inner side of the orbit-the lower, because