82 EXPERIMENTS, &c. 



shadow ought to move in the same direction 

 with the finger, which is the very reverse of 

 the fact to be explained. But as arguments 

 against error may be infinitely extended, and 

 as only one solution of a phenomenon can be 

 true, the readiest way of exposing the insuf- 

 ficiency of others, is to exhibit that which is 

 just. 



This, in the present case, seems to lie upon 

 the very surface of optical knowledge, and has 

 already been given by others, of various forms 

 of the same fact. When the image of the flame 

 is seen in the mirror as a point, its rays must be 

 accurately collected to a focus in the retina ; 

 but when seen as a surface, this must necessarily 

 be attributed to their focus being either before 

 or behind it ; in either of which cases, they will 

 occupy a place upon that membrane of some 

 assignable dimensions. In the present instance, 

 their diffusion over a part of the retina, depends 

 on the focus being behind it; for the eye is 

 now, from a condition of the experiment, in a 

 more relaxed state than it was just before, when 

 the rays of the same object were brought there 

 accurately to a point. The rays, therefore, 

 which go to the right side of the enlightened 

 surface of the retina, or picture as I shall call 

 it, are those which enter the eye at the right 



