EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 135 



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 deter- 

 mining 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 

 extremity, 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. 



Theory of the Perception of Colour. 



Opticians have long been divided on this point ; those who trusted to 

 popular notions and their own impressions adopting some theory of three primary 

 colours, while those who studied the phenomena of light itself proved that no 

 such theory could explain the constitution of the spectrum. Newton, who was 

 the first to demonstrate the actual existence of a series of kinds of light, 

 countless in number, yet all perfectly distinct, was also the first to propound 

 a method of calculating the effect of the mixture of various coloured light ; 

 and this method was substantially the same as that which we have just 

 verified. It is true, that the directions which he gives for the construction 

 of his circle of colours are somewhat arbitrary, being probably only intended 

 as an indication of the general nature of the method, but the method itself 

 is mathematically reducible to the theory of three elements of the colour- 

 sensation*. 



* See Note III. For a confirmation of Newton's analysis of Light, see Helmholtz, Pogg. Ann. 

 1852; and Phil. May. 1852, Part ir. 



