THEORIES OF COLOUR VISION 585 



1. The results of colour mixture are all adequately explained. Since to each of 

 the fundamental colours there is a corresponding sensation, and since mixtures of the 

 fundamentals can produce the whole range of colour, it follows that corresponding 

 stimulation of the sensations and their resynthesis in the brain fits in with the facts. 



2. The various classes of colour blindness which show abnormal types of luminosity 

 curve, abnormal colour mixture values, and possibly also a shortening of the spectrum, 

 are readily explained by supposing one of the sensations to be defective or absent. 

 For example, cases which show a shortening of the red end of the spectrum are stated 

 by the theory to have a deficient red sensation. The luminosity curve calculated on 

 this- basis is found to fit closely the curve found by experiment in these cases of colour 

 blindness. The hypothesis would appear therefore to be able to fully explain the 

 various cases which fall in this class. Certain objections have however been advanced 

 which it would be well to examine. ' (a) That it does not explain why the red blind and 

 the green blind state that the ends of the spectrum as they see them are yellow and 

 blue, whereas it would be expected that they would say green and blue if red blind, and 

 red and blue if green blind. An explanation of this behaviour can be readily obtained 

 by examining the forms of the red and green sensation curves, Fig. 299, for in both red 

 and green blindness a yellow light is that which stimulates the remaining sensation most 

 strongly, without at the same time involving the blue. In both types of case therefore 

 both red and green are regarded as being but degraded yellows, and the spectrum is 

 therefore named accordingly, (b) That the hypothesis does not explain why these 

 same cases call white white, instead of bluish green or purple. 



This is explained by the fact that a colour-blind person will call white what his 

 fellows who have normal colour vision call white, because he has learned his colour names 

 from them. In the same way a green-blind person will not call the leaves of a tree by 

 a wrong colour, although he will readily err if a piece of paper of the same colour as a 

 leaf be handed to him. 



The various types of colour blind which have normal luminosity curves cannot 

 be explained by the hypothesis without some further elaboration. As I have indicated 

 however they fit in well with the supposition that it is not the eye but the higher centres 

 which are at fault. The impulses which travel up the optic nerve are in every way 

 normal ; the error occurs in their interpretation. This would appear to be a reasonable 

 explanation which fits in with the other postulates of the hypothesis. It has been 

 advanced by Edridge Green as part of another hypothesis of colour vision, which will 

 be given later. 



3. Contrast, after images, and allied phenomena have not been adequately dealt 

 with. Helmholtz regarded contrast as an error of judgment, but Hering showed con- 

 clusively that such could not be the case. McDougall's hypothesis, which is to a large 

 extent founded on that of Young, will be found to add the features that are required 

 for the explanation of after images and contrast. 



HERING'S HYPOTHESIS states that there are in the retina three substances 

 which are all the time tending to dissociate into their components. They are however 

 either replaced or built up again from substances in the blood, as quickly as they are 

 destroyed. There is therefore equilibrium between anabolism and catabolism, when 

 the eye is unstimulated, and while this is the case no nerve impulses travel to the brain. 

 Now each of these substances is dissociated by one of the following colours, red, yellow, 

 white ; and is built up when green, blue or black fall on the retina. Thus one substance 

 will break up when red light falls on it, and will recombine when green does. There 

 is thus a red-green, a yellow-blue, and a white-black substance. When a coloured 

 light falls on the retina these three substances are broken down or are built up in varying 

 amounts and corresponding impressions sent to the brain. Tested by experiment this 

 view is found to acquit itself as follows : 



(1) The results of colour mixture are readily explained, with the possible exception 

 of the formation of grey, by the simultaneous anabolism and catabolism of one and the 

 same substance. 



