AS PERCEIVED BY THE EYE. 281 
Hence pe must be situated at a point such that w is the centre of gravity of 
‘65 pe+'55 U+'12 EG. 
To find it, we begin by determining 6 the centre of gravity of 55 U+-12 EG, 
then, joining 6 w, the point we are seeking must lie at a certain distance on the 
other side of w from c. This distance may be found from the proportion, 
65 : (55+°12):: Bw: wpe 
which determines the position of pe. The proper coefficient, by which the ob- 
served values of PC must be corrected is &, or 1:97. 
We have thus determined the position and coefficient of a colour by a single 
experiment, in which it was made to produce a neutral tint along with two of 
the standard colours. As this may be done with every possible colour, the 
method is applicable wherever we can obtain a disc of the proposed colour. In 
this way the diagram (No. 1) has been laid down from observations made in day- 
light, by a good eye of the ordinary type. 
It has been observed that experiments, in which the resultant tint is neutral, 
are more accurate than those in which the resulting tint has a decided colour, as 
in experiment (3), owing to the effects of accidental colours produced in the eye 
in the latter case. These experiments, however, may be repeated till a very 
good mean result has been obtained. 
But since the elements of every colour have been already fixed by our pre- 
vious observations and calculations, the agreement of these results with those 
calculated from the diagram forms a test of the correctness of our method. 
By experiment (No. 3), made at the same time with (1) and (2), we have 
39 PC +-21 U+-40 Bk=-59 V+-41 EG ititgchat pelea ae 
Now, joining U with pc, and V with EG, the only common point is that at 
which they cross, namely ‘y. 
Measuring the parts of the line V EG, we find them in the proportion of 
‘58 V and -42 EG=1-00 y 
Similarly, the line U pc is divided in the proportion 
‘78 pe and :22 U=1:00 y 
But °78 pce must be divided by 1:97, to reduce it to PC, as was previously ex- 
plained. The result of calculation is, therefore, 
39 PC +:22 U+:39 Bk="58 V+-42 EG 
the black being introduced simply to fill up the circle. 
This result differs very little from that of experiment (3), and it must be re- 
collected that these are single experiments,made independently of theory, and 
chosen at random. 
