530 THE SENSORIAL FUNCTIONS. 



The peculiar construction of the organ of 

 vision allows of our distinguishing the effects of 

 impressions made on particular parts of the 

 retina from those made on the rest, and from 

 their general effect on the whole surface. These 

 partial variations of sensibility in the retina give 

 rise to the phenomena of ocular spectra, as they 

 are called, which were first noticed by BufFon, 

 and afterwards more fully investigated by Dr. 

 Robert Darwin. A white object on a dark 

 ground, after being viewed steadfastly till the 

 eye has become fatigued, produces, when the eye 

 is immediately directed to another field of view, 

 a spectrum of a darker colour than the surround- 

 ing space, in consequence of the exhaustion 

 of that portion of the retina on which its image 

 had been impressed. The converse takes place, 

 when the eye, after having been steadfastly 

 directed to a black object on a light ground, 

 is transferred to another part of the same field ; 

 and in this case a bright spectrum of the object 

 is seen. 



It is a still more curious fact that the sensi- 

 bility of the retina to any particular kind of 

 light, may, in like manner, be increased or 

 diminished, without any change taking place 

 in its sensibility to other kinds of light. Hence 

 the spectrum of a red object appears green ; 

 because the sensibility of that portion of the 

 retina, on which the red image has been im- 

 pressed, is impaired with regard to the red rays, 



