SENSATIONS OF COLOR 547 



darkness of any purely colorless sensation, accordingly, is determined by the 

 ratio in which the intensity of katabolism stands to that of anabolism. 



Hering assumes four fundamental colors: red, yellow, green and blue. 

 These colors are selected because they can occur without any tinge of another 

 color occurring with them; or if they do exhibit any evident inclination 

 toward another color, it is never toward more than one other at the same 

 time. For example, yellow can merge into red or into green, but not into 

 blue; blue only into red or green; red only into yellow or blue. 



On the other hand, red and green are never clearly discernible in a color 

 at the same time, nor yellow and blue. That is, the presence of an evident 

 red sensation excludes that of an evident green; the presence of blue, that of 

 yellow, and vice versa; consequently, Hering calls these mutually exclusive 

 colors antagonistic colors. 



Just as the sensations of white and black are conditioned upon opposite 

 processes taking place in the white or black substance, the antagonistic colors 

 are produced by anabolism or katabolism, as the case may be, in two other 

 visual substances assumed by Hering, namely, the red-green- and the yellow- 

 blue-perceiving substances. Eed and yellow arise by katabolism, green and 

 blue by anabolism. 



The main proposition of Bering's theory therefore is this : the fundamental 

 sensations of the visual substances are grouped in three pairs : black and white, 

 yellow and blue, red and green. For each of these three pairs there is a cor- 

 responding anabolic and katabolic process of special quality. 



Since the amount of anabolism or katabolism caused by a light stimulus 

 in one of the three visual substances depends not only upon the intensity of 

 the stimulus, but also upon the excitability of the visual substance, the same 

 mixture of light may appear bright or dull colored, or colorless, according to 

 the physiological condition of the visual organ. 



When the visual organ has been protected from the light long enough so 

 that a condition of balance between the anabolic and the katabolic processes 

 is reached, and a colored light of moderate intensity is then admitted, the 

 excitability for that particular color will decrease until it is less than that 

 for the antagonistic color. Every mixed light which had previously appeared 

 colorless will now be seen with a tint of the antagonistic color, or if before 

 a mixture of fundamental colors was seen, it will now appear as a mixture 

 of the two antagonistic colors. Bering's theory can thus account for the 

 successive induction of color or color contrast. 



Agreeably to his theory, Hering reduces all color blindness to red-green 

 and yellow-blue blindness. Those who are blind to red and green lack the 

 red-green visual substance : everything which others see as red or green, they 

 see devoid of color ; in all mixed colors containing red or green, they see only 

 the yellow or blue, etc. 



E. SIMULTANEOUS CONTRAST 



The idea of simultaneous contrast can be most simply presented by means 

 of one or two concrete examples. If small colored sectors be placed on a 

 white disk, as in Fig. 239, and the middle point of each sector be interrupted 

 by a black and white strip, then when the disk is rotated one ought really to 



