480 JAMES CLERK MAXWELL. 



proportions the red of (24) and the green of (46), while 

 every tint lying between (48) and (64) could be produced 

 by a proper mixture of the green of (48) and the blue of 

 (64). Hence if a colour diagram be -constructed as explained 

 on page 473, the whole of the spectrum between the red of 

 (24) and the green of (46) will lie in a straight line, form- 

 ing one side of the triangle, while that lying between (48) 

 and (64) forms another side (the included angle being not 

 quite closed). It was also found that the violet beyond 

 (68) lay very nearly on the line joining (68) and (24), so 

 that the spectrum exhibited in this diagram "forms two 

 sides of a triangle, with doubtful fragments of the third 

 side." Thus " all the colours of the spectrum may be com- 

 pounded of those which lie at the angles of this triangle." 

 A diminished copy of one of Maxwell's lecture diagrams, 

 painted with his own hand and deduced (it is believed) 

 from experiments with the colour top, is given in Plate I. 1 



By studying Plate I., it will be seen that a mixture 

 of yellow and blue lights produces varying shades of dirty 

 yellow and^pinK, as the relative amount of blue is increased, 

 and by properly selecting the yellow and blue a neutral 

 gray (or white) may be obtained. 



The (pink tint produced by mixing blue and yellow 

 lights had feen previously noticed by Helmholtz and others. 

 The well-known fact that an admixture of blue and yellow 

 pigments produces generally a green was explained by Helm- 

 holtz on other grounds. Suppose, for instance, that we are 

 provided with properly chosen blue and yellow glasses : let- 

 sunlight shine through the blue glass and fall upon a screen, 

 and allow a second beam of sunlight, after reflection at a 

 mirror, to pass through the yellow glass and fall on the same 

 portion of the screen as the blue light. If the two colours 



1 The following table gives the position on the scale and wave 

 length in Fraunhofer's measure of the colours selected as standards : 



Scale Wave length. 



R Scarlet . .- ' . 24 2328 



G Green . . . 46f 1914 



BBlue . . :. 641 1717 



