No. 486] THE PROBLEM OF COLOR VISION 375 



up by blue light. As shown in the figure, orange is a mixed sensa- 

 tion due to the simultaneous partial destruction of the red-green 

 and the yellow-blue substances. Yellowish green and greenish 

 blue are likewise mixtures, and violet is supposed to combine the 

 partial construction of the yellow-blue with the destruction of 

 the red-green, the latter being indicated by the broken line. There 

 are four pure sensations, red, yellow, green, and blue. Color 

 blindness may be due to the absence or deficiency of the red-green 

 substance (protanopia and deuteranopia, the two forms being 

 varieties of a single type), or to lack of the yel]ow-l)lue substance 

 (tritanopia). Ilering further considered that there was a white- 

 black substance, built up in darkness to give rise to the sensation 

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