T>04 DR WILLIAM PEDDIE ON A 



Thus write 



360B=(112+p)W+(248-2?)Bk ^>>20 



360G=(110+g)W+(250-2)Bk 2 >20 



180 B + 180G = (120 + r)W + (240-r)Bk r> 7 



and we get 



j, + 2 = 18 + 2?'. 



Say, then, r = 3, ^> = 1 1, 9=13, and we get 



360G=123W+237Bk (I) 



360B = 123W + 237Bk (II) 



180G + 180B = 123W + 237Bk (Ill) 



Similarly write — 



174R + 186G = (87 + «)W + (273-a)Bk, «,j> 4 

 75 R+285 B = (95 + /3) W + (265- 8)Bk, /3> 19 



and we get, by elimination of B, and the use of equations (I) and (II), 



|8- 0-431 o= 12 



If, therefore, we assume a = 2, fl = 13, we have 



174E + 186G- 89W + 271Bk. . . (IV) 

 75E + 285 B = 108W+252Bk . . . (V) 



These equations agree (accidentally, to some extent, no doubt) with the following, 

 which was determined by a single observation : 



180 R + 180 B = 133 R+60 W + 167 Bk. 



Of course the equations (IV) and (V) would give, in conjunction with (I) or (II), 

 an expression for red in terms of a mixture of black and white. They must not be used 

 in that way, because of the fact that green and blue light, though they are equivalent to 

 grey in regard to colour sensation, are not equivalent to a mixture of black and white 

 in regard to complementariness to red. 



The yellow disc was also matched with a mixture of black and white, thus 



360Y = 295W+ 65 Bk j , VI) 



360Y = 258W + 102Bki 



