OTHER THEORIES 291 



sensations depend solely upon the relationship of the three underlying 

 metabolic processes. He considers that the light does not act directly 

 upon the achromatic and chromatic substances. There are materials 

 present in the retina white, red, green, etc. materials which are 

 chemically altered by the action of the light. The chemical action is 

 reversible under different conditions, thus resembling Hering's A and 

 D changes. It is the modifications in these peripheral substances that 

 are transmitted to the cerebral substrata of the actual sensations. 



In addition to four chromatic retinal processes, red, yellow, green, 

 blue, at the periphery, there are six central " values," the red process 

 exciting the red, yellow, and white values, the green exciting the green, 

 blue, and black values, and the blue exciting the blue, red and black 

 values. A yellow stimulus excites the red and yellow processes, thereby 

 ex citing the red, yellow, green, and white central values, of which the 

 red and green neutralise one another. By adding another link in the 

 psychophysical chain many more possible explanations are available 

 for the phenomena of colour vision. Thus, the red material has a yellow 

 valency, and the total yellow valency is made up of two parts, a direct 

 and an indirect, and so on. 



The reasoning throughout is very involved, but one feature stands 

 out clearly, viz., the division of the psychophysical processes into two 

 parts, a peripheral and a central. 



VII. EDRIDGE-GREEN'S THEORY 



Edridge-Green's theory is thus summarised in his own words. 



' A ray of light impinging on the retina liberates the visual purple 

 from the rods and a photograph is formed. The rods are concerned 

 only with the formation and distribution of the visual purple, not with 

 the conveyance of light impulses to the brain. There are cases in which 

 the visual purple is differently constituted and is not sensitive to certain 

 rays at one or both ends of the spectrum. The decomposition of the 

 visual purple by light chemically stimulates the ends of the cones (very 

 probably through the electricity which is produced), and a visual 

 impulse is set up which is conveyed through the optic nerve fibres to 

 the brain. If it were possible, in a case in which the spectrum appeared 

 of similar length and brightness to both, for a normal-sighted person and 

 a colour-blind one to exchange eyes, the normal-sighted would still see 

 colours properly and the colour-blind would still be colour-blind. The 

 character of the impulse set up differs according to the wave-length of 



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