282 



MR J. CLERK MAXWELL ON COLOUR 



Experiments made at Cambridge, with all the combinations of five colours, 

 show that theory agrees with calculation always within 0*012 of the whole, and 

 sometimes within 0002. By the repetition of these experiments at the numerous 

 opportunities which present themselves, the accuracy of the results may be ren- 

 dered still greater. As it is, I am not aware that the judgments of the human 

 eye with respect to colour have been supposed capable of so severe a test. 



Further consideration of the Diagram of Colours. 



We have seen how the composition of any tint, in terms of our three standard 

 colours, determines its position on the diagram and its proper coefficient. In the 

 same way, the result of mixing any other colours, situated at other points of the 

 diagram, is to be found by taking the centre of gravity of their reduced masses, as 

 was done in the last calculation (experiment 3). 



We have now to turn our attention to the general aspect of the diagram. 



The standard colours, V, U, and EG, occupy the angles of an equilateral tri- 

 angle, and the rest are arranged in the order in which they participate in red, 

 blue, and green, the neutral tint being at the point w within the triangle. If we 

 now draw lines through w to the different colours ranged round it, we shall find 

 that, if we pass from one line to another in the order in which they lie from red 

 to green, and through blue back again to red, the order will be — 





Coefficient. 





< 'oefficient. 



Carmine, . 



0-4 



Pale Chrome 



2-0 



Vermilion, 



10 



Mixed Green (U C), . 



0-4 



Red Lead, 



1-8 



Brunswick Green, 



0-2 



Orange Orpiment, 



1-0 



Emerald Green, 



1-0 



Orange Chrome, 



1-6 



Verditer Blue, . 



0-8 



Chrome Yellow, 



1-5 



Prussian Blue, . 



0-1 



Gamboge, . . 



1-8 



Ultramarine, 



1-0 



It may be easily seen that this arrangement of the colours corresponds to that 

 of the prismatic spectrum ; the only difference being that the spectrum is deficient 

 in those fine purples which lie between ultramarine and vermilion, and which 

 are easily produced by mixture. The experiments necessary for determining 

 the exact relation of this list to the lines in the spectrum are not yet completed. 



If we examine the colours represented by different points in one of these lines 

 through w, we shall find the purest and most decided colours at its outer ex- 

 tremity, and the faint tints approaching to neutrality nearer to w. 



If we also study the coefficients attached to each colour, we shall find that 

 the brighter and more luminous colours have higher numbers for their coefficients 

 than those which are dark. 



In this way, the qualities which we have already distinguished as hue, tint, 

 and shade, are represented on the diagram by angular position with respect to w, 

 distance from w, and coefficient ; and the relation between the two methods of 

 reducing the elements of colour to three becomes a matter of geometry. 



