SIK G. STOKES^ BART., ON THE PERCEPTION OP COLOUR. 259 



but merely as regards the colour sensation v/liicli it produces 

 — may be expressed by the equation — 



C = aX + hY + cZ, 



where " = " means matches in colour and intensity ; 

 " + " means superposed on ; and " — " (in case any of the 

 co-efficients should be negative), means that the term must 

 be transferred to the other side of the equation. ]\fatlie- 

 maticians will understand that, but I will not go further 

 into it. That equation represents the direct result of 

 observation ; and moreover different persons arrive at 

 results as to the mixture of colours very nearly agreeing 

 with one another, if we except persons belonging to the 

 somewhat rare, but by no means uncommon class, called 

 " colour-blind." But I will not go into the subject of 

 colour bhndness, it would take me too far from the subject 

 I have to bring before you, and therefore [ will content 

 myself by merely mentioning it. 



Now it would be a natural extension of this law, which 

 has been so carefully verified by Maxwell, and I may 

 mention, by others also, to assume that if you could get 

 at the supposed three primary sensations of colour, pure 

 by themselves, the same law would apply to the mixture 

 of those one with another. In this manner the subject 

 of the effect of mixing colour may be rendered very clear 

 in a general sort of way by means of what is called the 

 triangle of colours, but that would take me a little too 

 much into mathematics, very simple as those are, and I just 

 refer to it in passing. 



NoAv what supposition can we make physically as to these 

 three supposed primary sensations of colour? What laws 

 must any theory obey that we may make, respecting the 

 manner in which those sensations are produced ? Set 

 aside for a moment the existence of colour at all, and 

 think only of light. We know that we see separately a 

 vast amount of independent objects in the field of view. 

 There is a very wonderful structure in the retina of the 

 eye, correspondhig with that capacity we have of distin- 

 guishing one point in the field of view from another. In 

 the back of the retina there is a most remarkable structure, 

 in Avhich the nerves or nerve-fibres which are concerned 

 in vision end, which is called the bacillary hiyer. It 

 consists of very peculiar bodies of two different forms 

 in most eyes — in the human eye, for instance — which are 



