ll-M THE THEORY OF COLOURS IN RELATION TO COLOUR-BLINDNESS. 



colours which appear alike to the Colour-Blind. All these 

 lines either pass through one point or are parallel, ac- 

 cording to the standard colours which we have assumed, 

 :ind the other arbitrary assumptions we may have made. 

 Knowing this law of Colour-Blind vision, we may predict 

 any number of equations which will be true for eyes 

 having this defect. 



The mathematical expression of the difference between 

 Colour-Blind and ordinary vision is, that colour to the 

 former is a function of two independent variables, but to an ordinary eye, of 

 three ; and that the relation of the two kinds of vision is not arbitrary, but 

 indicates the absence of a determinate sensation, depending perhaps upon some 

 undiscovered structure or organic arrangement, which forms one-third of the 

 apparatus by which we receive sensations of colour. 



Suppose the absent structure to be that which is brought most into play 

 when red light falls on our eyes, then to the Colour-Blind red light will be 

 visible only so far as it affects the other two sensations, say of blue and 

 green. It will, therefore, appear to them much less bright than to us, and will 

 excite a sensation not distinguishable from that of a bluish-green light. 



I cannot at present recover the results of all my experiments ; but I recollect 

 that the neutral colours for a Colour-Blind person may be produced by com- 

 bining 6 degrees of ultramarine with 94 of vermilion, or 60 of emerald-green 

 with 40 of ultramarine. The first of these, I suppose to represent to our eyes 

 the kind of red which belongs to the red sensation. It excites the other two 

 sensations, and is, therefore, visible to the Colour-Blind, but it appears very 

 dark to them and of no definite colour. I therefore suspect that one of the 

 three sensations in perfect vision will be found to correspond to a red of the 

 same hue, but of much greater purity of tint. Of the nature of the other two, 

 I can say nothing definite, except that one must correspond to a blue, and the 

 other to a green, verging to yellow. 



I hope that what I have written may help you in any way in your 

 experiments. I have put down many things simply to indicate a way of thinking 

 about colours which belongs to this theory of triple sensation. We are indebted 

 to Newton for the original design ; to Young for the suggestion of the means 

 of working it out ; to Prof. Forbes* for a scientific history of its application 



*Phil. Mag. 1848. 



