ON THE THEORY OF COMPOUND COLOURS. 443 



six colours may be deduced from two equations, of which the most convenient 



form is 



V. U. G. B. W. Y. 



+397 +26-6 +337 -227 -77'3 =0 (17). 



-62-4 +18-6 -37'6 +457 +357 = (18). 



The value of D, as deduced from a comparison of these equations with the 

 colour-blind equations, is 



M98V + 0-078U-0-276G = D (19). 



By making D the same thing as black (B), and eliminating W and Y 

 respectively from the two ordinary colour-equations by means of D, we obtain 

 three colour-blind equations, calculated from the ordinary equations and con- 

 sistent with them, supposing that the colour (D) is black to the colour-blind. 



The following Table is a comparison of the colour-blind equations deduced 

 from Mr Simpson's observations alone, with those deduced from my observations 

 and the value of D. 



TABLE c. 



By (18) and (19) . . -13'6 +38-5 -86-4 +61-5 



The average error here is 1'9, smaller than the average error of the indi- 

 vidual colour-blind observations, shewing that the theory of colour-blindness being 

 the want of a certain colour-sensation which is one of the three ordinary colour- 

 sensations, agrees with observation to within the limits of error. 



In fig. 11, Plate VII. p. 444, I have laid down the chromatic relations of these 

 colours according to Newton's method. V (vermilion), U (ultramarine), and G 

 (emerald-green) are assumed as standard colours, and placed at the angles of 

 an equilateral triangle. The position of W (white) and Y (pale chrome-yellow) 

 with respect to these are laid down from equations (17) and (18), deduced 

 from my own observations. The positions of the defective colour, of white, and 

 of yellow, as deduced from Mr Simpson's equations alone, are given at " d," 

 "w," and "y." The positions of these points, as deduced from a combination 



562 



