224 THE HUMN BODY 



9 



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

 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 distinctly seeing distant objects, 

 might conceivably be accommodated for near vision in several 

 ways. The refracting indices of its media might be increased; 

 that of course does not happen; the physical properties of the 

 media are the same in both cases: or the distance of the retina 



FIG. 87. Diagram illustrating the surfaces at which light is refracted in the 

 eye. 



from the refracting surfaces might be increased, for example, by 

 compression of the eyeball by the muscles around it; however, 

 experiment shows that changes of accommodation can, by stim- 

 ulating the third cranial nerve, be brought about in the fresh 

 excised eyes of animals from which the muscles lying outside the 

 eyeball have been removed, in which no such compression is 

 possible; we are thus reduced to the third explanation, that the 

 refracting surfaces, or some of them, become more curved, and 

 so bring diverging rays sooner to a focus. Observation shows 

 that this is what actually happens: the corneal surface remains 

 unchanged when a near object is looked at after a distant one, 

 but the anterior surface of the lens becomes considerably more 

 convex and the posterior slightly so. When light meets the sep- 

 arating surface of two media some is reflected and some refracted. 



