256 RECENT PROGRESS OF THE THEORY OF VISION. 



sions of the colour in question. 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 accord- 

 ingly 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 satu- 

 rated 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 un- 

 fatigued 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 accumu- 

 late further details, to understand which it would be 

 necessary to enter upon lengthy descriptions of many 

 separate experiments. 



We have already seen enough to answer the question 

 whether it is possible to maintain the natural and innate 

 conviction that the quality of our sensations, and espe- 

 cially our sensations of sight, give us a true impression of 

 corresponding qualities in the outer world. It is clear 

 that they do not. The question was really decided by 

 Johannes Miiller's deduction from well ascertained facts 

 of the la of specific nervous energy. Whether the rays 



