THEORIES OF COLOR VISION 131 



in such a manner as to produce various gradations of achromatic lumi- 

 nosity in the visual sensation ranging from black, which is associated 

 with the absence of excitation, to a white associated with the maximum 

 rate of decomposition. In stage II of differentiation, the substance 

 presents to the action of light two separate parts, corresponding, for 

 example, " with two different radicals involved in the constitution of 

 its original complex molecules." One of these parts is decomposed at a 

 maximum rate by yellow light, and the other by blue light. The prod- 

 ucts of decomposition act upon the optic nerve to produce the colors 

 yellow and blue in consciousness. Simultaneous and equivalent decom- 

 position of the two parts of the substance, however, generates a gray in 

 consciousness. Stage III of evolution of the substance involves a 

 differentiation of the yellow-sensitive component into red-sensitive 

 and green-sensitive constituents. When these are acted upon separately, 

 they produce a psychologically primary red and green in consciousness, 

 but when simultaneously and equally decomposed they yield the original 

 value. 



Hecht [1928] objects to the assumption that the sensations yellow 

 and white are unique. He points out that if Young's idea is correct 

 then yellow is a phenomenon which is produced where red and green 

 receptor substances respond simultaneously. Similarly, white is 

 identified when all three — red, green, and blue — receptor substances 

 respond simultaneously. Both Hering and Ladd-Franklin have devised 

 theories that assume the existence of separate receptor substances for 

 yellow and for white. 



Hecht raised the question as to which of these two conceptions of 

 yellow and white is correct. His answer was obtained by means of a 

 simple experiment of binocular color mixing. He placed a red filter 

 (Wratten 29) in front of one eye and a green filter (Wratten 58) in 

 front of the other eye, and then viewed a brightly illuminated white 

 surface about 20 cm square placed on a black background. He found 

 that with one eye open the surface appears red ; with the other open, it 

 appears green; with both eyes open, the surface appears yellow. In 

 this experiment red light falls on a part of the retina of one eye and 

 green light falls on the corresponding portion of the retina of the other 

 eye, and the result is a yellow sensation; hence, Hecht concludes that 

 only Young's theory is tenable. If the green and red Wratten filters 

 are replaced by yellow (16) and blue (44A), a reasonably good white is 

 produced binocularly. 



The binocular formation of yellow and white shows that theories 

 which require special sensitive substances in the retina for yellow and 

 white are untenable. 



