THE EYE 345 



ball back of the iris is a ring of muscular tissue. When this contracts, 

 it constricts the iris and relaxes the capsule enclosing the crystalline 

 lens. The lens becomes thicker. The eye is then able to focus upon 

 nearby objects. If the eye wishes to focus on distant objects, the 



This side is in a state of relaxation This side is accommodated for near 

 and is for viewing objects at a objects. Why is the lens thick? 



distance. Why is the lens thin? 



FIG. 223. Diagram of an eye in the process of accommodation. 



opposite happens; the muscles relax, the iris expands and the lens 

 becomes flattened. In old people the crystalline lens becomes 

 hardened and does not adjust itself for different distances, thus 

 making it necessary for many elderly people to wear two kinds of 



FIG. 224. A concave lens spreads the light rays. 



glasses; one pair for near work and the other for seeing distant 

 objects. 



If a person reads for a long time the muscles of the eye grow 

 tired because of holding the lens in a certain position. We often 

 notice the results of tiring the muscles when we suddenly raise the 

 eyes from a book to some distant object. It seems to take a 



