THE EYE 339 



615. Accommodation. Rays of light coming from a dis- 

 tant object are less diverging than when coming from one 

 near by ; then the lens does not need to bend them so much 

 as in seeing objects near by. Adjusting the lens of the 

 eye to near or far vision is called accommodation. When 

 the eye muscles are at perfect rest, the eye is accommo- 

 dated to see clearly at all distances over twenty feet. So 

 distant vision requires no effort. When one wishes to see 

 an object less than twenty feet away from the eye, the 

 muscles must cause the lens to become more curved. 

 Thus the eye can see clearly up to about five inches from 

 the eye. Vision is best when the object is about ten 

 inches from the eye. 



Diagram of the eye in far sight. 



The lens b does not bring the rays from a point of light a together soon enough. 

 So the rays fall over the whole surface of the retina from d to e, making a confused 

 image instead of a clear point. When the rays spread less apart, as when the light 

 is moved farther away, to/ the lens brings them together sooner. Then the rays 

 fall upon a single point of the retina at^, and thus form a clear image. 



616. Far sight At the age of about forty-five, the muscles of 

 the lens lose some of their power of contraction and are unable to 

 make the lens so curved as in youth. Then the eye cannot be adjusted 

 for near vision, while for far vision the sight is as good as ever. So an 

 old man holds his newspaper at arms' length. He also aids the action 

 of his lens by placing before his eye a spectacle lens which corrects 

 the deficiency in the lens of his eye. 



617. Near sight. In young people the lens often brings the 

 rays together too soon. The rays must be made more diverging by 



