498 



THE HUMAN BODY. 



without any effort; all rays emanating from a point of the 

 object meet again in one point on the retina. 

 Accommodation. Points on near objects send into the eye 

 [verging rays: these therefore would not come to a focus on 



o o J -> 



the retiiui but behind it, and would not be seen distinctly, 

 did not some change occur in the eye; since we can see them 

 quite plainly if we choose (unless they be very near indeed), 

 there must exist some means by which the eye is adapted 

 or accommodated for looking at objects at different distances. 

 That some change does occur one can, also, readily prove 



FIG. 131. Diagram illustrating the surfaces at which light is refracted in the 

 eye. 



by observing that we cannot see distinctly, at the same 

 moment, both near and distant objects. For example 

 standing at a window, behind a lace curtain, we can if we 

 choose look at the threads of the lace or the houses across 

 the street; but when we look at the one we only see the 

 other indistinctly; and if, after looking at the more distant 

 object, we look at the nearer we experience a distinct sense 

 of effort. It is clear, then, that something in the eye is 

 different in the two cases. The resting eye, suited for dis- 

 tinctly seeing distant objects, might conceivably be accom- 

 modated for near vision in several ways. The refracting 



