278 MR J. CLERK MAXWELL ON COLOUR, 
gas-light instead of day-light. The tints which before appeared identical will now 
be manifestly different, and will require alteration, to reduce them to equality. 
Thus, in the case of carmine, we have by day-light, 
44 C+:22 U+'34 EG='17 SW+°83 Bk 
while by gas-light (Edinburgh) 
47 C+:08 U+-45 EG=:25 SW +°75 Bk 
which shows that the yellowing effect of the gas-light tells more on the white 
than on the combination of colours. If we examine the two resulting tints which 
appeared identical in experiment (3), observing the whirling discs through a 
blue glass, the combination of yellow, blue, and black, appears redder than the 
other, while through a yellow glass, the red and green mixture appears redder. 
So also a red glass makes the first side of the equation too dark, and a green 
glass makes it too light. 
The apparent identity of the tints in these experiments is therefore not real, 
but a consequence of a determinate constitution of the eye, and hence arises the 
importance of the results, as indicating the laws of human vision. 
The first result which is worthy of notice is, that the equations, as observed 
by different persons of ordinary vision, agree in a remarkable manner. If care 
be taken to secure the same kind of light in all the experiments, the equations, 
as determined by two independent observers, will seldom show a difference of 
more than three divisions in any part of the equation containing the bright stand- 
ard colours. As the duller colours are less active in changing the resultant tint, 
their true proportions cannot be so well ascertained. The accuracy of vision of 
each observer may be tested by repeating the same experiment_at different times, 
and comparing the equations so found. 
Experiments of this kind, made at Cambridge in November 1854, show that 
of ten observers, the best were accurate to within 1} division, and agreed within 
1 division of the mean of all; and the worst contradicted themselves to the ex- 
tent of 6 degrees, but still were never more than 4 or 5 from the mean of all 
the observations. 
We are thus led to conclude— 
1st, That the human eye is capable of estimating the likeness of colours with 
a precision which in some cases is very great. 
2d, That the judgment thus formed is determined, not by the real identity of 
the colours, but by a cause residing in the eye of the observer. 
3d, That the eyes of different observers vary in accuracy, but agree with each 
other so nearly as to leave no doubt that the law of colour-vision is identical for 
all ordinary eyes. 
