104 JAMES CLEUK MAXWELL 



Maxwell adopted as his three standard colours: 

 red, of about wave length 0,302 ; green, wave length 

 5,281 ; and violet, 4,569 tenth metres. On the scale 

 of Maxwell's instrument these are represented by the 

 numbers 24, 44 and tj8. 



Let us take three points A, B, ( ' at the -comers 

 of an equilateral triangle to represent on a diagram 

 these three colours. The position of any other colour 

 on the diagram will be found by taking weights 

 proportional to the amounts of the colours A, B, (' 

 required to make the match between A, H, (' and the 

 given colour: these weights are placed at. A, H, (' 

 respectively ; the position of their centre of gravity 

 is the point required. Thus the position of white is 

 given by the equation 



\v = is-r, (24) + 31 -j (if) + ;HK (<;s) 



which means that weights proportional to]S(, :{|'4 

 and .'U)5 are to bo placed at A, 11, (' respectively, 

 and their centre of gravity is to 'be found. The point 

 so found is the position of white. Any other colour 

 is given by the equation 



X = a (24) + /, (44) + c(<>8). 



Again, the position on the diagram for all colours 

 for which ", />, c are all positive lies within the 

 triangle A H C. If one of the eo-ctlicicnts, say /, is 

 negative the same construction applies,' but the 

 weight applied at (' must be treated as acting 

 in the opposite direction to those ait, A and H. 

 A mixture of the given colour and (' matches a 

 mixture of A and K It is clear that the point 

 corresponding to X will then lie outside the triangle 



