380 COORDINATION AND SENSATION 



This change, called accommodation, occurs in the crystal- 

 line lens. 1 In the process of accommodation, changes 

 occur in the shape of the crystalline lens, as follows : 



1. In looking from a distant to a near object, the lens 

 becomes more convex, i.e., rounder and thicker (Fig. 161). 

 This change is necessary because the greater divergence 

 of the light from the near objects requires a greater con- 

 verging power on the part of the lens. 2 



2. In looking from near to distant objects, the lens 

 becomes flatter and thinner (Fig. 161). This change is 

 necessary because the less divergent waves from the dis- 

 tant objects require less converging power on the part of 

 the lens. 



The method employed in changing the shape of the lens is difficult 

 to determine and different theories have been advanced to account for 

 it. The following, proposed by Helmholtz, is the theory most generally 

 accepted : 



The lens is held in place back of the pupil by the suspensory liga- 

 ment. This is attached at its inner margin to the membranous capsule, 

 and at its outer margin to the sides of the eyeball, and entirely sur- 

 rounds the lens. It is drawn perfectly tight so that the sides of the 

 eyeball exert a continuous tension, or pull, on the membranous capsule, 

 which, in its turn, exerts pressure on the sides of the lens, tending to 

 flatten it. This arrangement brings the elastic force of the eyeball into 

 opposition to the elastic force of the lens. The ciliary muscle plays 

 between these opposing forces in the following manner: 



To thicken the lens, the ciliary muscle contracts, pulling forward 

 the suspensory ligament and releasing its tension on the membranous 



1 Wiih respect to its adjustments the eye does not differ in principle from vari- 

 ous other optical instruments, such as the microscope, telescope, photographer's 

 camera, etc., which, in their use, form images of objects. These all require some 

 adjustment of their parts, called focusing, which adapts them to the distance. 

 The eye's method of focusing, however, differs from that of most optical instru- 

 ments, in that the adjustment is brought about through changes in the curvature 

 of a lens. 



2 The converging power of convex lenses varies as the curvature the greater 

 the curvature, the greater the converging power. 



