OTHER THEORIES 285 



IV. Schenck's Theory 



Schenck's theory^ is founded upon the Young-Helmholtz theory. 

 From the psychological point of view each simple or pure light-sensation 

 is held to depend upon an independent physiological process. The 

 simple light-sensations are white, red, green, yellow and blue. Since 

 all sensations can be synthesised out of red, green and blue, but not out 

 of white or yellow and two other simple sensations, red, green and blue 

 are the three fundamental sensations. Their physiological counterparts 

 are three " visual substances." 



The difficulty that white and yellow have no simple physiological 

 counterparts is explained developmentally. In an earlier phase of 

 development the photopic apparatus, the cones, contained only one 

 visual substance, the stimulation of which gave the sensation white, 

 and which was nearly allied to the scotopic visual substance of the rods. 



The white substance first underwent a " panchromatising " change 

 whereby it became more sensitive to lights of long wave-length. It 

 next became differentiated into two substances, a yellow substance, 

 specially sensitive to rays of long wave-length, and a blue substance, 

 specially sensitive to rays of short wave-length. These substances 

 retain the peculiarity of the mother-substance that when they are 

 simultaneously stimulated with strengths which correspond to the 

 effect of ordinary daylight they give rise to the sensation white. 



Analogously the yellow substance became differentiated into a 

 red and a green substance, which when equally and simultaneously 

 stimulated arouse the sensation yellow. 



These changes may be represented diagrammatically thus : 



Regional Effects. I. In the photopic normal eye : (a) the centre 

 of the retina is trichromatic, (b) the middle peripheral zone is red-green- 

 blind, (c) the outer peripheral zone is totally colour-blind. 



II. The limit of the red-green-blind zone is only relative : it 

 extends further peripherally with increase of size, intensity, and satura- 

 tion of the observed object. 



III. Colour matches valid for the centre are valid for the periphery. 



IV. Luminosity matches valid for the centre are valid for the 

 periphery. 



' Arch.f. d. ges. Physiol, cxvm. 129, 1907. 



