ON THE THEORY OF COMPOUND COLOURS. 417 



By compounding this resultant tint with some other colour, we may find the 

 position of a mixture of three colours, at the centre of gravity of its components ; 

 and by taking these components in different proportions, we may obtain colours 

 corresponding to every part of the triangle of which they are the angular points. 

 In this way, by taking any three colours we should be able to construct a 

 triangular portion of Newton's diagram by painting it with mixtures of the three 

 colours. Of course these mixtures must be made to correspond with optical 

 mixtures of light, not with mechanical mixtures of pigments. 



Let us now take any colour belonging to a point of the diagram outside 

 this triangle. To make the centre of gravity of the three weights coincide with 

 this point, one or more of the weights must be made negative: This, though 

 following from mathematical principles, is not capable of direct physical inter- 

 pretation, as we cannot exhibit a negative colour. 



The equation between the three selected colours, x, y, z, and the new colour 

 u, may in the first case be written 



u = x+y + z (1), 



x, y, z being the quantities of colour required to produce u. In the second case 

 suppose that z must be made negative, 



u = x + y z (2). 



As we cannot realize the term z as a negative colour, we transpose it to the 

 other side of the equation, which then becomes 



u + z = x + y (3), 



which may be interpreted to mean, that the resultant tint, u + z, is identical 

 with the resultant, x + y. "We thus find a mixture of the new colour with one 

 of the selected colours, which is chromatically equivalent to a mixture of the 

 other two selected colours. 



When the equation takes the form 



u = x-y-z (4), 



two of the components being negative, we must transpose them thus, 



u + y + z = x (5), 



which means that a mixture of certain proportions of the new colour and two 



of the three selected, is chromatically equivalent to the third. We may thus in 



all cases find the relation between any three colours and a fourth, and exhibit 



VOL. I. 53 



