THE LENS 



259 



through their nerves, so that any object upon which we 



direct our attention is automatically brought into focus. 



Flexibility of lens. In childhood, the lens is very 



flexible and can be made so convex as to focus objects 



FIG. 140. Diagram illustrating the effect of a convex lens upon rays of light : A, 

 when parallel rays are brought to a focus (principal focus) \ B, when rays from 

 a near object are brought to a focus beyond the principal focus ; C, when rays 

 from an object nearer than the principal focus continue divergent. 



but three or four inches from the eye. As one grows 

 older, the lens gradually loses its flexibility and tends 

 to become rigid in its flattest form. As a result, we 

 gradually lose the power of seeing near objects clearly 

 because their light rays are not brought to a focus 

 on the retina. Ultimately, between the ages of thirty- 

 five and fifty years, a normal person is usually unable 

 to read or thread a needle without strain. He is there- 

 fore forced to aid his eyes by the use of glass lenses, 

 which by their convexity assist the cornea and lens in 



