THE EYES 



237 



Parallel Rays fo- 

 cused on the 

 Retina. 



rays of light. Otherwise they would reach the retina 

 without coming to a focus at all. 



Rays of light from a distance enter the eye in 

 parallel lines, but the natural convex shape of the 

 crystalline lens in a normal eye brings 

 them to a focus on the retina. Rays 

 of light from small objects do not enter 

 the eye in parallel lines, but an effort 

 of the muscles is necessary to make 

 the crystalline lens convex enough to 

 bring these divergent rays to a focus 

 on the retina. The eye has muscles 

 that do the important work of making 

 this lens become more convex when it is 

 necessary. The more convex a lens is, 

 whether it is the natural one in the eye, or a glass 

 lens in spectacles, the greater is its power to bend 

 divergent rays to meet at a point. 



When a normal eye looks at distant objects, these 

 muscles are at rest, because the convexity of the crys- 

 talline lens is sufficient to focus the rays of light on 

 the retina. In looking at near objects, however, 

 as in reading, writing, sewing, and every kind of work 

 requiring close examination, these muscles are brought 

 into use. Although they are involuntary (that is, 

 they work without our willing it), effort is required 

 to keep the lens adjusted for work which has to be 

 held close to the eyes. On this account, it is restful 

 to the eyes when using them for such work, oc- 



