THE EYE 



double convex. 1 Because of the great difference in density 

 between the air on the outside and the aqueous humor 

 within, the cornea is the more powerful of the two. The 

 crystalline lens, however, performs a special work in 

 focusing which is of great importance. The iris also aids 

 in focusing since it, through the pupil, regulates the amount 

 of light entering the back chamber of the eyeball and 

 causes it to fall in the center of the crystalline lens, the 

 part which focuses most accurately. 



Accommodation. A difficulty in focusing arises from 

 the fact that the degree of divergence of the light waves 



DISTANT 



FIG. 161. Diagram showing changes in shape of crystalline lens 

 to adapt it to near and distant vision. 



entering the eye from different objects, varies according 

 to their distance. Since the waves from any given point 

 on an object pass out in straight lines in all directions, 

 the waves that enter the eye from distant objects are at 

 a different angle from those that enter from near objects. 

 In reality waves from distant objects are practically parallel, 

 while those from very near objects diverge to a consider- 

 able degree. To adjust the eye to different distances 

 requires some change in the focusing parts that corre- 

 sponds to the differences in the divergence of the light. 



1 Consult some work on physics on the different kinds of lenses and their 

 uses. 



