74 THE FOUNDATIONS OF SCIENCE 



or by mixing in the eye pairs of colours which lie 

 opposite each other in our diagram red and green, 

 blue and yellow, greenish blue and orange. It must 

 be noted that, in all these cases, rays of light must 

 be mixed so as to be blended when they enter the 

 eye. The mixture of pigments or paints outside the 

 eye is another and more complicated story. 



As we have said, it is probable that the extent 

 and intensity of all sensations differ from person to 

 person, possibly from race to race, on the average 

 of large numbers of individuals. About four per 

 cent, of European men are colour-blind to red and 

 green, and see those colours as grey. Another much 

 rarer type of human being seems to be blind to blue 

 and yellow, while some others are totally colour-blind. 



The common red-green blindness indicates that 

 red and green are primary colour sensations, and so 

 far all are agreed. But two explanations of the whole 

 of the phenomena are held: according to Hering's 

 theory, there are four primaries red, green, blue, 

 and yellow; while, on Young and Helmholtz's 

 theory, all colour sensations are compounded of red, 

 green, and blue, as figured in the triangle we used to 

 illustrate another subject in Chapter I. 



Primitive races seem to be much less sensitive to 

 blue and rather less sensitive to yellow than Euro- 

 peans, as though these sensations had not yet fully 

 developed. It is interesting to note that in the 

 Iliad of Homer colour terminology, except for red, 



