DARK SPOT ON THE RETINA, ETC. 99 



pression upon this part of the eye, a very strong light 

 does, and even when we use candles or highly luminous 

 bodies in place of wafers the body does not wholly dis- 

 appear, but leaves behind a faint cloudy light, without, 

 however, giving anything like an image of the object 

 from which the light proceeds. 



When the objects are white wafers upon a black ground, 

 the white wafer absolutely disappears, and the space which 

 it covers appears to be completely black ; and as the light 

 which illuminates a landscape is not much different from 

 that of a 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 

 surround it, and the consequence of this is, that when the 

 wafer disappears, the spot which it 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 



* 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. 



