ON THE THEOBY OF COMPOUND COLOURS. 427 



The position of the slit X was then shifted from (24) to (28), and when 

 the proper adjustments were made, I found a second colour-equation of this form 



Oct. 18, J. 16 (28) + 21 (44) + 37 (68) = W (14). 



Subtracting one equation from the other and remembering that the figures in 

 brackets are merely symbols of position, not of magnitude, we find 



16(28) = 18'5 (24) + 6 (44) (15), 



shewing that (28) can be made up of (24) and (44), in the proportion of 18'5 

 to 6. 



In this way, by combining each colour with two standard colours, we may 

 produce a white equal to the constant white. The red and yellow colours from 

 (20) to (32) must be combined with green and blue, the greens from (36) to (52) 

 with red and blue, and the blues from (56) to (80) with red and green. 



The following is a specimen of an actual series of observations made in this 

 way by another observer (K.) : 



TABLE III. 



Oct 13, 1859. Observer (K.). 



(A"> (Y) (Z) 



18J(24) + 32i(44) + 32 (68) = W*. 



17J(24) + 32J(44) + 63 (80) = W. 



18 (24) + 32J(44) + 35 (72) = \V. 



19 (24) + 32 (44) + 31(68) = W*. 



19 (24) + 30J(44) + 35 (64) = W. 



20 (24) + 23 (44) + 39 (60) = W. 



21 (24) + 14 (44) + 58 (56) = W. 



22 (24) + 62 (52) + 11 (68) -W. 



22 (24) + 42 (48)+29J(68) = W. 



19 (24) + 31$(44) + 33 (68) = W*. 



16 (24) + 28 (40) + 32(68) = W. 

 6 (24) + 27 (36) + 32J(68) = W. 



23 (32)+llJ(44) + 32J(68) = W. 



17 (28) + 26 (44) + 32J(68) = W. 



20 (24) + 33J(44) + 32J(68) = W*. 

 46 (20) + 33 (44) + 30 (68) = W. 



The equations marked with an asterisk (*) are those which involve the 

 three standard colours, and since every other equation must be compared with 

 them, they must be often repeated. 



542 



