102 JAMES CLERK MAXWELL 



In this way wo can form a number of equations 

 for all possible colours, and if wo like to take any 

 three colours A, B, (' as standards, we obtain a result 

 which may bo written generally 



x X = a A + I 15 -f C. 



or x parts of X can be matched by <t parts of A, 

 combined with It parts of 15 and c parts of C. If the 

 sign of one of the quantities re, />, or < is negative, it 

 indicates that that colour must be combined with X 

 to match the other two. 



Xow Maxwell was able to show that, if A, B, C 

 be properly selected, nearly every other colour can 

 be matched by positive combinations of these 

 three*. These three colours, then, are primary colours, 

 and nearly every other colour can be matched by a 

 combination of the three primary colours. 



Experiments, however, with coloured discs, such 

 as were undertaken by Young, Forbes and Maxwell, 

 were not capable of giving satisfactory results. The 

 colours of the discs were not pure, spectrum colours, 

 and varied to some extent with the nature of the 

 incident light. It was for this reason that Ilelmholtx 

 in 1852 experimented with the spectrum, and that 

 Maxwell about the same time invented his colour 

 box. 



The principle of the latter was very simple. Sup- 

 pose we have a slit S, and some arrangement for 

 forming a pure spectrum on a screen. Let there 

 now be a slit R placed in the red part of the spectrum . 

 on the screen. When light, falls on the slit S, only 

 the red ravs can reach R, and hence conversely, if the 



