304 Report of the Committee on Colour- Vision. 



for colour- 

 blindness 

 induced by 

 disease by the 

 Young-Helni- 

 holtz tlieory. 



Hering's 

 theory and 

 colour-blind- 

 ness induced 

 by disease. 



ing to this theory, the mixed sensations of red, green, and 

 violet produce the sensation of white light; but evident^ in 

 the cases where colour is absent in every part of the spectrum 

 except in the blue — the rest being seen as white — some 

 different explanation is required. Or again, if we take into 

 account the fact that at a certain distance from the centre of the 

 retina all sensation of colour, varying according to its luminosity 

 and its hue, is lost, though light is still seen, the ordinary appli- 

 cation of the theory cannot be insisted upon. 



It may seem that Hering's theory is fully capable of explaining 

 most of these phenomena, but there are facts against its accept- 

 ance which are very weighty. For instance, according to this 

 theory, the sensations of red and green, and of yellow and 

 blue, ought always to be present together, but in some cases 

 of colour-blindness caused by over-use of tobacco, and atrophy of 

 the optic nerve, the blue is the only colour sensation felt, the 

 yellow being absent from that part of the spectrum in which it 

 should be present. Again, when the intensity of the light 

 producing the spectrum is reduced the sensation of red disappears 

 long before that of green, which shows that the two sensations 

 are not always co-existent. The shortened spectrum of what are 

 called the red-blind is also opposed to the theory, for the 

 luminosity of the green is proportionally much greater to them 

 than the red than it is to the green-blind. 



Shift of the 

 neutral point, 

 in the spec- 

 trum caused 

 by different 

 qualities of 

 white light. 



Note (c). 



The neutral point of the spectrum will vary in all cases 

 of colour-blindness according to the whiteness of the light 

 with which the spectrum is compared. Even to the normal 

 eye there is a ray near the yellow which can match very 

 closely indeed the light of a gas lamp or candle, though there 

 is none which matches the whiteness of ordinary day- or 

 sun-light. Now a match made by the normal eye of a 

 coloured light with some ray of the spectrum will be equally 

 a match to the colour-blind of either type, since in both the 

 colour and its match in the spectrum the same one sensation 

 will be absent. It therefore follows that their neutral point, with 

 a candle or oil lamp as a standard of whiteness, must be the same 

 yellow ray, but to the red-blind this ray would appear greenish 

 if compared with the white of day-light, and to the green-blind 

 reddish. If the mental picture of white light were that of 

 day-light, then evidentty the green signal light would have to 

 be much bluer to the colour-blind than to the normal eye, to 

 prevent a confusion between it and their neutral colour than 

 would have to be the case when lamp-light is the mental image 

 of white light. In testing a large number of men by lamp- 

 light it was invariably found that its light was always called 

 yellow or orange by the normal-eyed, and we may therefore 

 suppose that the general idea of whiteness is derived from 



