THE SENSATION OF SIGHT. 225 



some point of the white sheet near the margin of the black, and 

 after 30 to 60 seconds draw the black sheet quickly away, with- 

 out losing sight of the point. The half of the white sheet 

 which is then exposed appears suddenly of the most brilliant 

 brightness ; and thus it becomes apparent how very much the 

 first impression produced by the upper half of the sheet had 

 become blunted and weakened, even in the short time taken by 

 the experiment. And yet, what is also important to remark, 

 the observer does not at all notice this fact, until the contrast 

 brings it before him. 



Lastly, it is possible to produce a partial fatigue of the 

 retina in another way. We may tire it for certain colours only, 

 by exposing either the entire retina, or a portion of it, for a 

 certain time (from half a minute to five minutes) to one and the 

 same colour. According to Young's theory, only one or two 

 kinds of the optic nerve fibres will then be fatigued, those 

 namely which are sensitive to impressions of the colour in ques- 

 tion. All the rest will remain unaffected. The result is, that 

 when the after-image appears, red, we will suppose, upon a grey 

 background, the uniformly mixed light of the latter can only 

 produce sensations of green and violet in the part of the retina 

 which has become fatigued by red light. This part is made red- 

 blind for the time. The after-image accordingly appears of a 

 bluish green, the complementary colour to red. 



It is by this means that we are able to produce in the retina 

 the pure and primitive sensations of saturated colours. If, for 

 instance, we wish to see pure red, we fatigue a part of our retina 

 by the bluish green of the spectrum, which is the complementary 

 colour of red. We thus make this part at once green-blind and 

 violet-blind. We then throw the after-image upon the red of as 

 perfect a prismatic spectrum as possible ; the image immediately 

 appears in full and burning red, while the red light of the 

 spectrum which surrounds it, although the purest that the world 

 can offer, now seems to the unfatigued part of the retina less 

 saturated than the after-image, and looks as if it were covered 

 by a whitish mist. 



These facts are perhaps enough. I will not accumulate fur- 



I. Q 



