behind the lens, while that of the distant objects falls with almost parallel lines. To return to the camera-obscura, you must draw out the tube with the lens, in other words, remove the latter farther from the intercepting plate if nearer objects are to be impressed, and on the other, push it in, if more distant ones are wanted. The same effects might be produced at equally the same distance by simply substituting lenses of different power. Now, the human eye has to fulfil the requirement of projecting clearly-defined images on the retina, whether they are but a few inches off, or at an immeasurable distance. The eye being strictly subject to lenticular laws, either the space between the lens and the retina must have the power of varying, or the lens itself, by a change of form, must exercise now a stronger now a weaker power of refraction.
Fig. 4.
The same parts as in Fig. 2, besides: I, Iris; K, Cillary Muscle; L, Suspensory Ligament of the lens (zonula).
The conclusion has now been arrived at, that this power of accommodation depends on a varying curve of the lens.[1]
In order to effect this, a great degree of elasticity, chiefly of the outer capsule, was obviously necessary, and we find this requisite complied with by an admirably delicate structure of concentric layers, according to which its density reaches the minimum at the periphery of the lens, while its aggregate power of refraction is increased, as if it were composed entirely of the strong refracting substance of which the centre consists.
As the power of accommodating its focus rests on this quality of the lens, it is necessarily accompanied by a loss of elasticity with increasing age. The eye of a man of sixty, that sees distinctly at a dis-
- ↑ The mode of procedure is now known down to the last and most minute detail. The surfaces of the lens give back extremely delicate reflections, which with the proper aids may be measured in the living eye, and from the size of which the curve may be calculated as in convex and concave mirrors. As auxiliary of this change of form, there is a peculiar agent—a muscle embedded in the choroid (K, Fig. 4)—which has the power of contracting and expanding the suspensory ligament of the lens.
