530 COLOUR SENSATIONS, [BOOK m. 



mitting three distinct kinds of impulses ; or we may suppose that 

 the visual substances are three in number instead of six, the 

 changes in each substance provoking a primary sensation. 



Such are the two main theories of colour vision ; and much may 

 be said in favour of both of them ; at the same time both of them 

 present many difficulties. To discuss them fully, is a task beyond 

 the limits of this book, and to discuss them in any but a full manner 

 would be unsatisfactory. We must be satisfied therefore with the 

 foregoing simple statement of the two views. Independently of any 

 theory, however, we may remember (1) that all the sensations 

 which we experience under the action of light of whatever kind 

 may be reduced to six, white, black, red, yellow, green and blue ; 

 and (2) that these may be all reproduced by various mixtures of 

 three standard sensations, if black be allowed to indicate the 

 absence of all sensation. These are matters of fact : what is at 

 present debated is whether the six fundamental sensations are 

 the outcome of three primary sensations or whether they represent 

 six distinct conditions of the visual apparatus. 



Colour Blindness. Persons vary much in their power of 

 appreciating and discriminating colour, i.e. in the intensity and 

 accuracy of their colour sensations; and some people regard 

 as similar, colours which to most people are glaringly distinct ; 

 these latter are said to be ' colour blind.' The most common form 

 of colour blindness is that of persons unable to distinguish green 

 and red from each other. As in the case of Dalton, they tell a red 

 gown lying on a green grass plot, or a red cherry among the green 

 leaves, by its form, and not by its colour. They confound not only 

 red, green, and certain forms of brown, but also rose, purple, 

 and blue. Such persons are often spoken of as ' red blind.' On the 

 Hering theory they lack the red-green visual substance; hence, 

 all the colour sensations they possess must be those of yellow and 

 blue freed from all mixture of red or green ; and such accounts as 

 have been given of their sensations by those persons who are 

 'red-blind' in one eye, but possess normal vision with the other, 

 accord with this conclusion. On the Young-Helmholtz theory, 

 such persons lack the primary red sensation; and hence the 

 sensations which they have must be mixtures of green and blue 

 alone, our yellow appearing to them a bright green, and our green- 

 blue a kind of grey. 



All such red-blind people ought, on either theory, to be 

 less affected than are persons with normal eyes, by the red end 

 of the spectrum : this ought with them to be shortened and obscure. 

 In a certain number of persons who confound red and green, this is 

 the case ; but in some instances no such lack of appreciation of the 

 red end of the spectrum can be ascertained. Such cases have been 

 supposed to be green-blind, that is lacking the primary sensation 

 of green. According to the Hering theory green blindness apart 



