72 
PEOFESSOE J. CLEEK MAXWELL OX 
to be carefully observed. There are twenty observations of this equation, the mean of 
which gives 
18-6(24)+31'4(44) + 30-5(68)=W* (16.) 
as the standard equation. 
We may use the twenty observations of this equation as a means of determining the 
relations between the errors in the different colours, and thus of estimating the accu- 
racy of the observer in distinguishing colours. 
The following Table gives the result of these operations, where E stands for (24), 
G for (44), and B for (68) 
Table V.- 
(K+G+B)=2-67 
Mean Errors in the Standard Equation. 
-v/G^+B^-1-67 
-v/B^+E^=1-26 
-s/E^+G^=l-33 
x/E^+GP+B^=l-76 
(E)= *54 
(G~B)=-99 
(G+B)=2-3I 
(G)=I-22 
(B-E)=-85 
(B-fE)=I-59 
(B)=I-I5 
(E-G)=-86 
(E-l-G)=I-67 
The first column gives the mean difference between the observed value of each of the 
colours and the mean of all the observations. The second column shows the average 
error of the observed differences between the values of the standards, from the mean 
value of those differences. The third column shows the average error of the sums of 
two standards, from the mean of such sums. The fourth column gives the square root 
of the sum of the squares of the quantities in the first column. I have also given 
the average error of the sum of E, G and B, from its mean value, and the value of 
V^E^+G^-fB^ 
It appears from the first column that the red is more accurately observed than the 
green and blue. 
§ IX. Belative Accuracy in Observations of Colour and of BnghtQiess. 
If the errors in the different colours occurred perfectly independent of each other, 
then the probable mean error in the sum or difference of any tAvo colours Avould be the 
square root of the sum of their squares, as given in the fourth column. It will be seen, 
however, that the number in the second column is always less, and that in the third 
always greater, than that in the fourth ; showing that the errors are not independent of 
each other, but that positive errors in any colour coincide more often with positive thaji 
with negative errors in another colour. Now the hue of the resultant depends on the 
ratios of the components, while its brightness depends on their sum. Since, therefore, 
the difference of tAvo colours is always more accurately observed than their sum, varia- 
tions oi colour are more easily detected than variations in brightness, and the eye appears 
to be a more accurate judge of the identity of colour of the tAVO parts of the field than 
of their equal illumination. The same conclusion may be draAvn from the value of the 
