532 COMPARATIVE PHYSIOLOGY. 



ACCOMMODATION OF THE EYE. 



Using the material already referred to, the student may 

 observe that, with the natural eye of the albino rabbit, its 

 lens (or better that of a bullock's eye, being larger), or a bi- 

 convex lens of glass, there is only one position of the instru- 

 ments and objects which will produce a perfectly distinct image. 

 If either the eye (retina), the lens, or the object be shifted, in- 

 stead of a distinct image, a blurred one, or simply diffusion- 

 circles, appear. 



A photographer must alter either the position of the object 

 or the position of his lens when the focus is not perfect. The 

 eye may be compared to a camera, and since the retina and 

 lens can not change position, either the shape of the lens must 

 change or the object assume a different position in space. As 

 a matter of fact, any one may observe that he can not see 

 objects distinctly within a certain limit of. nearness to the eye, 

 known as the near point {punctum proximum) ; while he be- 

 comes conscious of no effect referable to the eye until objects 

 approach within about sixty-five to seventy yards. Beyond the 

 latter distance objects are seen clearly without any effort. 



There are many ways in which we may be led to realize 

 these truths : 1. When one is reading a printed page it is only 

 the very few words to which the eyes are then specially di- 

 rected that are seen clearly, the rest of the page appearing 

 blurred ; and the same holds for the objects in any small room. 

 We speak of picking out an acquaintance in an audience or 

 crowd, which implies that each of the individuals composing 

 the throng is not distinctly seen at the same time. 2. If an ob- 

 server hold up a finger before his eyes, and direct his gaze into 

 the distance (relax his accommodation), presently he will be- 

 hold a second shadowy finger beside the real one — i. e., he sees 

 double ; his eyes being accommodated for the distant objects, 

 can not adapt themselves at the same time for near ones. 



In what does accommodation consist ? From experiments 

 it has been concluded that accommodation consists essentially 

 in an alteration of the convexity of the anterior surface of the 

 lens. 



This change in the shape of the lens is accomplished as 

 follows : The lens is naturally very elastic and is kept in a par- 

 tially compressed condition by its capsule, to which is attached 

 the suspensory ligament which has a posterior attachment to 



