THE COLLOIDAL STATE. 69 



green or yellow and blue cannot be perceived at the 

 same time, while black and white can be perceived simul- 

 taneously and can be mixed in different proportions in 

 the gray sensations. That we are dealing in this case 

 with two kinds of antagonistic processes has been suf- 

 ficiently emphasized by HERING. 



MACH has also pointed out in a general way the 

 essential difference between the two kinds of antagonistic 

 processes in his doctrine of the sensations of motion. 

 While criticising PLATEAU'S oscillation theory MACH 

 writes : 



" When two things, A and 5, are designated as positive 

 and negative with regard to each other, one understands 

 thereby that A can, through the addition of B, be in 

 part or entirely destroyed. This relation exists between 

 many sensations and their after-images, but not be- 

 tween all. It exists, for example, between the percep- 

 tion of a movement and its after-image, w r hich is an 

 entirely similar movement, but of an opposite character. 

 It does not exist, however, between the sensations black 

 and white, of which the one may also be the after-image 

 of the other. Both sensations are entirely different from 

 each other, and the two together do not annihilate each 

 other, but produce, as do two different colors, a mixed 

 color, namely, gray. In this case, therefore, the terms 

 positive and negative are not appropriately applied. " 



According to our conception, those antagonistic reactions 

 that behave as positive and negative values are homo- 

 drome, the other heterodrome reactions. 



Nothing stands in the way of regarding the individual 

 kinds of light as catalyzers of antagonistic reactions. 

 Corresponding with this ide.a, we would pronounce the 



