OTHER THEORIES 289 



Its weakest feature is the failure common to the three-components 

 theory and most of its variants to account satisfactorily for the facts 

 of induction. These are presumably relegated to the central nervous 

 organs as in the theory of zones. Schenck elaborates Ad. Fick's^ views 

 as to the peculiar position of yellow. He adopts Tschermak's division^ 

 of each visual substance into two parts, a stimulus-receptor or inter- 

 mediary, and a sensation-stimulator. 



The rod visual substance is the primitive visual substance : it 

 possesses a receptor or resonator for light of short wave-length, and a 

 slightly damped resonator for light of medium wave-length. 



The development of the cone visual substance from the primitive 

 rod substance is accompanied by increased damping of the resonator 

 for light of medium wave-length, and also panchromatisation, i.e., 

 development of a resonator for light of long wave-length. 



As regards the sensation stimulators there is a white stimulator in 

 the rods, and there are red, green, and blue stimulators in the cones. 



In the lowest developmental phase of the cones all the resonator 

 and all the stimulator molecules are intermingled, so that they act as 

 a physiological unit and are equivalent to a white substance. 



In the next phase the resonator for short waves becomes limited to 

 the blue stimulator and thus forms a blue substance, whilst the others 

 remain intermingled and are equivalent to a yellow substance. 



In the final phase each resonator is limited to its corresponding 

 stimulator. 



As has been seen the theory has to be strained to account for all the 

 facts of the different varieties of colour blindness, and it may well be 

 doubted whether the complications thus introduced make it more 

 acceptable than the ordinary three-components theory. 



V. Wundt's Photochemical Theory 



Wundt^, from the data available at the time, concluded that the 

 facts of colour-mixtures and so on did not necessitate a multiplicity of 

 specifically different stimulation elements or substances. His theory is 

 as follows. 



Two different stimulation processes are set in action by every 

 retinal excitation, a chromatic and an achromatic. The chromatic 

 excitation is a function of the wave-length of the light ; the achromatic, 



1 Arch, f d. ges. Physiol. XLvn. 275, 1890. * m^^ lxxxii. 589, 1900 



^ Orundziige d. physiol. Psychologie, i. 535. 1893. 



p. c. V. 19 



