382 PHYSIOLOGY CHAP. 



substances into two parts a stimulus-receptor (Reizevtvpf anger) 

 and a sensation-stimulator (IZmpfindungserreger). Schaum sug- 

 gested that the receptors act as optical sensitisers of the cones, 

 as visual purple is the sensitiser of the rods ; Kicharz, that they 

 may be regarded as optical resonators for light of short, medium, 

 or long wave-length. The mechanism by which the stimulators 

 transform the optic resonances into physiological stimuli of the 

 nerve-fibres is unknown, but when these are conducted to the 

 central nervous organs they give rise to achromatic and chromatic 

 sensations. The rods contain one stimulator, for white ; the cones 

 contain three, for red, green, and blue. 



Even with this modification the Young-Helmholtz theory does 

 not adequately explain why the sensations of white and yellow 

 which for physiological and psychological reasons are regarded by 

 Wundt and Hering as simple sensations are not connected with 

 as simple physiological processes as the sensations of red, green, 

 and blue. To remove this difficulty Schenck assumes that this 

 connection between the sensations of white and yellow by simple 

 physiological processes actually exists in the early phases of 

 development of the visual organs. 



According to Schenck, the cones which subserve the perception 

 of brightness or luminosity and which, on the well-established 

 view of v. Kries, constitute the only organ capable of colour-vision 

 contain at an earlier developmental phase one substance only, 

 which, on stimulation, gives the sensation white. This primitive 

 visual substance is allied to the visual substance of the rods, which 

 subserves scotopic vision, as it is little sensitive to light of long 

 wave-length. 



Later on, the primitive visual substance undergoes a change 

 which makes it more sensitive to light of longer wave-length, 

 which Schenck on analogy with photographic nomenclature- 

 calls panchromatisation, but it still continues to be the substrate 

 of white light vision. 



At a further stage of development there is a cleavage of the 

 primitive visual substance, and pari passu a differentiation of its 

 two parts and of their function. This cleavage occurs at two 

 distinct periods : 



(a) In the first place, two visual substances are formed from the 

 original white substance, one of which is specially sensitive to 

 vibrations of long wave-length, and gives rise to the sensation of 

 yellow; the other is more sensitive to vibrations of short wave- 

 length, and excites the sensation of blue. But when equally and 

 simultaneously stimulated, they still give rise to the sensation 

 white, as did the mother-substance before its differentiation into 

 two parts. 



(b) Later again, by an analogous process, the yellow substance 

 becomes differentiated into a red and a green substance, which 



