ASSOCIATION AND COLOR DISCRIMINATION 487 



Hering bases his theory of opponent colors on the sensations 

 of complementary colors. When certain wave-lengths of red 

 and green are presented to the human eye simultaneously, the 

 result is a colorless sensation. The simultaneous perception of 

 yellow and blue of certain wave-lengths produces the same effect. 

 These four color sensations, red, green, blue, and yellow, are 

 regarded by Hering as the primary ones from mixtures or 

 dilutions of which all other color sensations are derived. 

 To these are added the primary sensations, black and white, 

 which on mixture result in sensations of gray. This theory 

 maintains that there exists in the eye three distinct visual sub- 

 stances affected b}^ pairs of antagonistic physiological processes 

 in such a way as to produce the six primary sensations, white- 

 black, red-green, and yellow-blue. In favor of this theory is 

 claimed for consideration the color sensitivity of the part of the 

 normal human retina lying outside of the fovea. The extreme 

 periphery of the retina is color-blind. Within this totally color- 

 blind zone lies an area which is red-green blind, but is sensitive 

 to blue and yellow stimuli, while the center of the retina is sen- 

 sitive to all colors. The most common form of color-blindness 

 is that in which red and green are not discriminated, and cases 

 are known in which yellow and blue are confused. Either type 

 of color-blindness might be explained by the absence of one of 

 the physiological substances. The same explanation might be 

 applied to account for the failure of the sticklebacks in the ex- 

 periments described to discriminate between yellow and blue. 



The observation that on mixture red, green, and blue produce 

 white is the basis for the Young-Helmholtz theory, according to 

 which there are three primary colors instead of four from which 

 all other colors are derived. The wave-lengths of the red and the 

 blue are closely identical with the fundamental colors chosen by 

 the upholders of the four-color theory, while the green lies on 

 the yellow side of the green of Bering's theory. This theory 

 explains very well certain cases of partial color-blindness by as- 

 suming the absence or diminution of one of the three theoret- 

 ical components. If this theory accounts for the inability of 

 sticklebacks in these experiments to discriminate between blue 



