390 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



at which the mixture must be situated according to the mixing 

 laws, are the co-ordinates of the centre of gravity of masses 

 situated at the points at which the components are represented. 



It is clear that a diagram constructed on these principles will 

 vary in accordance with our choice of units and fixed points. Thus 

 Newton chose white as a fixed point and arranged the simple 

 colours at equal distances from it, so that his diagram was a 

 circle. "We should obtain this result except that the part of the 

 curve passing from violet to red must be a straight line, since purple 

 can only be mixed from these colours and therefore lies on the 

 chord joining them. 



Recurring to the experimental facts previously mentioned, we 



Green, 



YMow 



Orcuig* 

 Xal 



FIG. 4. Colour Table. 



see that the most convenient form of diagram will be that 

 figured (Fig. 4). From red to yellow the curve is a straight line 

 since we found that mixtures of colours between these limits 

 matched pure spectral lights ; we then get a sharp bend in the 

 part of the diagram beyond green, representing the low saturation 

 of mixtures from this region ; finally we have the straight line 

 through purple. 



Suppose now we select three colours in the spectral series, 

 e.g. red, green, blue, then in accordance with our previous de- 

 ductions, all mixtures of these are represented in the triangle 

 RGB. This includes a good deal of our complete diagram, but 

 not all of it. If we choose, instead of blue, violet, then the 

 triangle RGV comprises nearly the whole spectral diagram ; in 

 other words, nearly all colours can be matched by mixing three 

 chosen lights in suitable proportions (vid. infra, p. 404). There is 





