THE THEOEY OF COMPOUND COLOUES. 
71 
The equations marked wnth 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. 
The following Table contains the means of four sets of observations by the same 
observer (K): — 
Table IV. (K.) 
44-3(20) + 31-0(44)+27-7(68)=W. 
16T(28)+25-6(44)+30-6(68)=W. 
22-0(32)+12T(44)+30-6(68)=W. 
6-4(24)+25-2(36)+31-3(68)=W. 
15-3(24)+26-0(40) + 30-7(68)=W. 
19-8(24)+35-0(46)4-30-2(68)=W. 
21- 2(24) + 41-4(48)+27-0(68)=W. 
22- 0(24)+62-0(52)+13-0(68) = W. 
21-7(24)+10-4(44)+61-7(56)=W'. 
2d-5(24)+23-7(44)+40-5(60)=W. 
19-7('24) + 30-3(44)+33-7(64)=W. 
18-0(24) + 31-2(44)+32-3(72)=W. 
17- 5(24)4-30-7(44)+44-0(76)=W. 
18- 3(24)+33-2(44)+63-7(80)=W. 
§ Vm. JJetermmation of the Average Error in Observations of different kinds. 
In order to estimate the degree of accuracy of these observations, I have taken the 
differences between the values of the three standard colours as originally observed, and 
their means as given by the above Table. The sum of all the errors of the red (24) 
from the means, was 31 T, and the number of observations was 42, which gives the 
average error -74. 
The sum of eiTors in green (44) was 48’0, and the number of observations 31, giving 
a mean error 1‘55. 
The sum of the errors in blue (68) was 46-9, and the number of observations 35, 
ginng a mean eri’or 1T6. 
It appears therefore that in the observations generally, the average error does, not 
exceed 1'5; and therefore the results, if confirmed by several observations, may safely 
be trusted to that degree of accuracy. 
The equation between the three standard colours was repeatedly observed, in order to 
detect any alteration in the character of the light, or any other change of condition 
which would prevent the obseiwations from being comparable with one another ; and also 
because this equation is used in the reduction of all the others, and therefore requires 
L 2 
