DARK SPOTS ON THE RETINA, ETC. 13 



white wafer, we should expect, whether we use 

 one or both eyes,* to see a black or a dark spot 

 upon every landscape, within 15 of the point 

 which most particularly attracts our notice. The 

 Divine Artificer, however, has not left his work 

 thus imperfect. Though the base of the optic 

 nerve is insensible to light that falls directly upon 

 it, yet it has been made susceptible of receiving 

 luminous impressions from the parts which sur- 

 round it; and the consequence of this is, that 

 when the wafer disappears, the spot which is 

 occupied, in place of being black, has always the 

 same colour as the ground upon which the wafer 

 is laid, being white when the wafer is placed 

 upon a white ground, and red when it is placed 

 upon a red ground. This curious effect may be 

 rudely illustrated by comparing the retina to a 

 sheet of blotting-paper, and the base of the optic 

 nerve to a circular portion of it covered with a 

 piece of sponge. If a shower falls upon the 

 paper, the protected part will not be wetted by 

 the rain which falls upon the sponge that covers 

 it, but in a few seconds it will be as effectually 

 wetted by the moisture which it absorbs from the 

 wet paper with which it is surrounded. In like 

 manner the insensible spot on the retina is stimu- 

 lated by a borrowed light, and the apparent 

 defect is so completely removed, that its existence 

 can be determined only by the experiment already 

 described. 



Of the same character, but far more general in 



* When both eyes are open, the object whose image falls 

 upon the insensible spot of the one eye is seen by the other, 

 so that, though it is not invisible, yet it will only be half as 

 luminous and, therefore two dark spots ought to be seen. 



