70 
PEOFESSOE J. CLEEK 3IAXWELL OX 
may be considered complete, and the operator must measure the breadth of each sht by 
means of the wedge, as before described, and write down the result as a colour-equation, 
thus — 
Oct 18, J. 18-5(24)H-27(44) + 37(68)=W* (13.) 
This equation means that on the 18th of October the obser\"er J. (myself) made an obser- 
vation in which the breadth of the slit X was 18 ’5, as measured by the wedge, while its 
centre was at the division (24) of the scale; that the breadths of Y and Z were 27 and 
37, and their positions (44) and (68); and that the illumination produced by these slits 
was exactly equal, in my estimation as an observer, to the constant white W. 
The position of the slit X was then shifted from (24) to (28), and when the proper 
adjustments were made, 1 found a second coloui’-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)=18-6(24)+6(44), (15.) 
showing 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 111. 
Oct. 13, 1859. Observer (K). 
(X). (T). (Z). 
18|(24)H-32i(44) + 32 (68)=W*. 
17i(24)+32i(44)+63 (80)=W. 
18 (24) + 321(44) + 35 (72)=W. 
19 (24)+32 (44) + 31i(68)=W*. 
19 (24) + 30i(44) + 35 (64)=W. 
20 (24)+23 (44)+39 (60)=W. 
21 (24)+14 (44)+58 (56)=W. 
22 (24)+62 (52)+ll (68)=W. 
22 (24)+42 (48)+29i(68)=W. 
19 (24)+31i(44)+33 (68)== W^. 
16 (24)+28 (40)+32i(68)=:W. 
6 (24)+27 (36)+32i(68)=W. 
23 (32)+lli(44)+32i(68)=W. 
17 (28)+26 (44)+32i(68)=W. 
20 (24)+33i(44)+32i(68)=W*. 
46 (20)+33 (44)+30 (68)=W. 
