130 BIOPHYSICAL CHARACTERISTICS OF THE EYE 



The other two substances are supposed to be decomposed at a maxi- 

 mum rate by red and yellow lights, and recomposed by green and blue 

 lights. 



The six psychological primary colors, red, green, yellow, blue, white, 

 and black, are correlated directly with six distinctive rates of change 

 in the three basic photochemical substances, as shown in Table III— 5. 



This system of relationships satisfactorily explains the manner in 

 which the psychological primaries combine with one another, and 

 accounts especially for the " antagonistic " behavior of red and green, 

 and yellow and blue. Troland [1920] points out that the three weakest 

 points in this theory are: (1) the failure of opposite processes in the 

 black- white substance actually to cancel each other while those in the 

 other two substances always leave a residual gray; (2) the fact that 

 psychologically primary red and green do not in fact combine to pro- 

 duce gray, but rather a yellow; and (3) the failure of continuous stimula- 

 tion of a single region of the retina to reduce the sensory effects of all 

 stimuli to a neutral mid-gray. 



Apparently, therefore, the essentially antagonistic natures of the 

 processes underlying respective members of the three pairs of primary 

 colors are not supported by the experimental facts. 



To meet these and other criticisms, Mrs. Ladd-Franklin [1892] pro- 

 posed a theory based essentially on the existence of a single light- 

 sensitive substance located in the retina. She also assumed a gradual 

 evolution of the color sense of the retina from a primitive condition of 

 colorless vision such as still exists in the periphery of the retina to a 

 high degree of specialization by the fovea to reactions of color. Thus 

 the retina is supposed to preserve a complete record of the historical 

 changes of the anatomical development of rods into cones and also a 

 comparable development of the rod-pigment sensitizer. In man only 

 is there an additional intermediate cleavage stage, the visual yellow. 



According to the Ladd-Franklin theory a hypothetical light-sensitive 

 substance must be assumed to break down in three stages. In stage I 

 its decomposition by light leads to the initiation of nerve impulses 



