PHILOSOPHY OF ZOOLOGY. 



was brought into the optical axis, the image formed by the 

 rays which passed through it, was distinct, the image of the 

 rays of the central hole was rendered obscure, and the rays 

 of the remote lateral hole not entering the eye, only two 

 dots appeared. 



When I placed the dot at the distance of a foot from the 

 eye, and then interposed the perforated card, only one 

 image was formed, nor did more appear when the eye was 

 removed to a greater distance, or brought a short way with- 

 in the limits of distinct vision. In this case, the rays which 

 entered the different holes were nearly parallel, and conse- 

 quently converged at one point. 



When I placed the dot and the perforated card, as in 

 the first experiment, within the limit of distinct vision, 

 where three images appeared, and gradually removed the dot 

 beyond that limit, the three images still continued to ap- 

 pear even at that distance, where only one would have been 

 visible, had the perforated card been there interposed for 

 the first time. In like manner, when the dot, viewed 

 through the perforated card, beyond the limits of distinct 

 vision, and appearing single, is brought nearer the eye, it 

 will still appear single within that distance ; at which, if 

 viewed for the first time through the card, three images 

 would have been formed. These appearances are similar 

 to those which take place in the eye, in ordinary circum- 

 stances. We see a dot upon paper at a greater distance, if 

 first viewed at the limit of distinct vision, and then gradu- 

 ally withdrawn, than when the eye is directed towards it 

 for the first time in its remote station ; and the same thing 

 takes place with objects held close to the eye. In these 

 cases, the action in the retina is continued by a weaker im- 

 pression than is requisite for its first excitement. These 

 results warrant the conclusion, that the rays which do not 



