ON VISION. 91 



The combination of lenses, or humours in the eye, is 

 supposed to take off those prismatic colours that are 

 produced when rays of 'light are strongly and differently 

 refracted much in the same way that a similar effect is 

 produced by the compound object-glass in an achromatic 

 telescope ; and thus the eye, taken even as a piece of 

 mechanism, and without any reference to life, or the 

 faculty of sight, is equal, nay superior, to the utmost 

 effort of human contrivance. When we come to add 

 to it those natural powers of perception and adjustment 

 by which it acts and adapts itself, it would become, 

 were it not so common, and in the midst of a world 

 as wondrous, a great and constant wonder. The re- 

 fraction of rays that come from objects at different 

 distances, are different, and those which come from 

 a near one, approach each other more rapidly, and, 

 therefore, meet sooner than those that come from a 

 remote object. Light from objects at different dis- 

 tances, therefore, must meet in points at different 

 distances, behind the pupil of the eye. But vision is 

 not distinct, unless the point where the rays meet be 

 the very surface of the retina ; and, therefore, there 

 must be in the eye a power of altering its form, by 

 the motion of the retina backwards and forwards, by 

 an alteration in the convexity, or otherwise, of the 

 refractive power of the lenses, or by both ; and one 

 can easily feel such a power, by habituating the eyes 

 to look at objects at different distances. Looking 

 closely, together with the straining of the eye-lids, 

 which usually accompanies such an effort, seems to 

 increase the convexity of the lenses; for, when the 

 sight has begun to dazzle and fail at the usual reading 



