202 THE POPULAR SCIENCE MONTHLY. 



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 indo- 

 lent condition, is compressed between the two layers of the liga- 

 ment 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 volun- 



Fio. 3. VISUAL PORTION OF THE KETINA AS SEEN BY THE OPHTHALMOSCOPE ; 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.) 



tary, 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 



