﻿Theories in Relation to Colour-Blindness, 919 



both will appear identical in hue and colour. This proves 

 conclusively that the defect is not due to a diminution of a 

 hypothetical red sensation, because all the rays coming 

 through the blue-green glass are supposed to affect the red 

 sensation, and yet we have been able to correct the erroneous 

 match by the subtraction of red light. On the other hand, 

 there are colour-blind persons who, whilst disagreeing with 

 the normal white equation, agree with it when the com- 

 parison white is diminished in intensity. 



A totally erroneous view of a case may be obtained 

 through methods based on the three-sensation theory. A 

 man may be examined and found to put too much green in 

 his white equation ; he is therefore classified as partially 

 green-blind. Further examination shows that he can pass 

 the wool test, but fails to see a deep red light formed by 

 rays from the red end of the spectrum. The fact that he is 

 insensible to these rays explains the facts of his case, in- 

 cluding his error in colour mixing. As certain red rays are 

 invisible to him, these have to be subtracted from white 

 light. If his white could be seen by a normal-sighted 

 person, it w r ould appear greenish white. Therefore, if the 

 colours used in the white equation have their normal value, 

 he will put more green than normal in the equation, as he is 

 reallv matching a greenish white. 



Even the facts of colour mixing are far more satisfactorily 

 explained by a non-elemental than by an elemental theory. 

 For instance, a considerable amount of one spectral colour 

 may be added to another without altering its appearance. 



Houstoun has shown very clearly that from a mathematical 

 and physical point of view only one substance is necessary, 

 and that there is no evidence of more than one. All the 

 facts tend to show that the visual purple is the visual 

 substance, that the cones are the terminal perceptive visual 

 organs, and that the rods are not perceptive elements, 

 but are concerned with the formation and distribution of 

 the visual purple. Vision takes place by stimulation of the 

 cones through the photo-chemical decomposition of the 

 liquid surrounding them, which is sensitized by the visual 

 purple *. The ends of the cones being stimulated through 

 the photo-chemical decomposition of the visual purple by 

 light, a visual impulse is set up which is conveyed through 

 the optic-nerve fibres to the brain. The character of the 

 stimulus and impulse differs according to the wave-length 

 of the light causing it. In the impulse itself we have the 



* * The Physiology of Vision,' G. Bell & Sons, London, 1920, p. 134. 



