34 COLOUR VISION 



and of yellow of 580 fxjx causes a colour sensation which can be accurately 

 matched with that derived from some spectral colour between those 

 wave-lengths. The exact position, i.e. the wave-length, of this colour 

 depends upon the relative amounts of the two colours in the mixture. 

 If there is an excess of red the resultant mixture will match a colour 

 nearer the red than the yellow- green. Moreover, the position of the 

 colour will be accurately represented by the mass centre of the weights 

 of the two components, i.e. by the centre of gravity, as Newton showed.- 

 V. Kries^ gives the following matches of mixtures of 670"8 /x/x and 

 552 fi/ji. for the spectrum of gas light, the measurements being obtained 

 with the Helmholtz-Konig spectrophotometer : 



All the colour sensations in this region are therefore functions of a 

 single variable and can be represented on a straight line, AB, where A 

 represents about 800 /zyu and B 540 /u/u,. Each point on the line represents 



A C D B 



I \ 1 1 



Fig. 6. Representation of mixtures of lights of long wave-length on o. straight line. 



a colour sensation. If an amount, Wj, of light L^, a.t A is mixed with 

 an amount, m-., of light, L^., at B, then the resultant sensation is a light, 

 Xa, at C such that CA : CB as m. : Wj. Similarly, if ?n,j of L,.j at C be 

 mixed with m^ of L, at B, the resultant sensation is a light, L^, at D, 

 such that DC : DB as m^ : m-s, and so on. All mixtures therefore 

 which give a sensation corresponding to a given point give rise to the 

 same sensation or match accurately. Hence the range from 800 /x^ to 

 540 /u/x must, like the purple range, be represented in the colour diagram 

 as a straight line. 



o 



' Ztsch. f, Psychol, u. Physiol, d. iSinncsorg. xni. 281, 1896. 



