350 ADVANCED PHYSIOLOGY 



a second, at least, for the change to be made. This process 

 is called accommodation, and occurs almost entirely in the 

 lens, although the front of the cornea is also slightly 

 modified. 



The form which a lens must take when the object viewed 

 is near at hand, as compared with its form when the object is 

 far away, is shown in Figure 173. When a point is near the 

 lens, as at A, the rays of light going from it will diverge 

 rapidly. In order to bring them to a focus at the distance 

 of the screen or retina, they must be bent very decidedly 

 inward. It is also evident that if a point is farther away from 

 the lens, as at B, the rays will not be diverging as rapidly 

 when they reach the lens, and not being turned from their 

 course as much as those from A, will come to a focal point in 

 front of the screen or retina at 6. Moreover, we have already 

 noticed that the more bulging or convex the lens, the more 

 sharply the light rays going through it will be bent from their 

 paths. If, therefore, a flatter lens is substituted, like the one 

 marked I, which has a curvature only sufficient to bring rays 

 from point B f to a focus at b f , the rays from point A' will not be 

 sufficiently bent and will be focused behind the screen at a', 

 But if a more convex lens is substituted, like the one aft I', 

 the rays from both A f and B f will be focused in front of the 

 screen at a" and 6". Thus a lens of a definite curvature is 

 necessary for properly focusing light from any particular point. 

 If one wishes to look at a distant point after examining a 

 near one, the lens must be flattened; and when the attention is 

 turned from a distant point to a near one, a more convex 

 lens is necessary. 



Mechanism of Accommodation. In the eye it is not neces- 

 sary to replace one lens with another, for the lens of the eye 

 is not rigid like glass, but can change its shape from a thin, 

 flattish to a very convex form. Attached to the edge of the 

 lens and holding it to the choroid layer, we have noticed the 

 suspensory ligament. This is under tension, and as a result 



