474 JAMES CLERK MAXWELL. 



to produce a distinct yellow from a mixture of emerald green 

 and vermilion, but impossible to produce a green from a 

 mixture of blue and yellow. Any colour which can be 

 produced by a mixture of all the three primary tints will 

 lie within the triangle ; but a colour which must be mixed 

 with one of the primary tints in order to match a mixture 

 of the other two will lie without the triangle. Thus the 

 pale chrome referred to above must be mixed with blue in 

 order to match a mixture of red and green, and it must 

 therefore be placed in such a position that the centre of 

 gravity of the yellow and blue taken in proper proportions 

 may be on the line joining the red and green, and may 

 divide this line in the inverse ratio of the amounts of red 

 and green respectively required. A copy of Maxwell's dia- 

 gram, showing the chromatic relations of coloured papers, 

 is given in the accompanying plate (Plate II.) The original 

 is in the Cavendish Laboratory. 



As above mentioned, Maxwell found that in the case of 

 colour-blind persons only four discs (including black) were 

 required instead of five to ensure a match. The following 

 is an example of a colour-blind equation: 



where G-, B, and K represent green, blue, and red respect- 

 ively. To a normal eye the outer circle represented by the 

 left-hand side of the equation appears of a dark blue green, 

 but to the colour blind this matches the full red of the 

 smaller circle. 



By comparing different colour matches, it is possible to 

 determine the position on the colour diagram of the tint 

 corresponding to the missing sensation in the colour-blind. 

 Thus, if two colours appear to a colour-blind person to 

 match, they can only differ in their effect upon a normal 

 eye by the degree in which they excite the sensation which 

 is missing in the colour-blind, and they must therefore lie 

 upon a line passing through the position of this missing sen- 

 sation. By determining two such lines, the position of the 

 pure sensation is located. A line drawn through this point 

 and the position of white is, to a colour-blind person, a 



