Page:Popular Science Monthly Volume 45.djvu/216

 which this is done has been known only within the last few years. The lens is elastic, and in a quiescent, or what is called an indolent condition, is compressed between the two layers of the ligament which holds it in place. In this condition, when the rays from distant objects are practically parallel as they strike the eye, the lens is adjusted for infinite distance. When, however, we examine a near object, by the action of a little muscle within the eyeball the ligament is relaxed and the elastic lens becomes more convex. This action is called accommodation, and is voluntary,

 magnified about seven and a half diameters, showing the blood-vessels branching; from the point of entrance of the optic nerve, and the yellow spot surrounded by the dotted oval. (After Loring.)

though usually automatic. The fact that it is voluntary is illustrated by the very simple experiment of looking at a distant object through a gauze placed a few feet from the eye. When we see the distant object distinctly, we do not see the gauze; but by an effort we can distinctly see the meshes of the gauze, and then the object becomes indistinct. In some old persons the lens not only becomes flattened, but it loses a great part of its elasticity and the power of accommodation is nearly lost.

The changes in the curvatures of the lens in accommodation have been actually measured. The lens itself is only about a third