42 Prof. Maxwell’s Account of Experiments on 
the north, without curtains or any bright-coloured object near 
the window. The same combination was never made twice in 
one day, and no thought was bestowed upon the experiments ex- 
cept at the time of observation. Of course the graduation was 
never consulted, nor former experiments referred to, till each 
combination of colours had been fixed by the eye alone ; and no 
reduction was attempted till all the experiments were concluded. 
The coloured discs were cut from paper painted of the follow- 
ing colours :—Vermilion, Ultramarine, Emerald-green, Snow- 
white, Ivory-black, and Pale Chrome-yellow. They are de- 
noted by the letters V, U, G, W, B, Y respectively. These 
colours were chosen, because each is well distinguished from the 
rest, so that a small change of its intensity im any combination 
can be observed. Two discs of each colour were prepared, so 
that in each combination the colours might occasionally be 
transposed from the outer circle to the imner. 
The first equation was formed by leaving out vermilion. The 
remaining colours are Ultramarine-blue, Emerald-green, White, 
Black, and Yellow. We might suppose, that by mixing the 
blue and yellow in proper proportions, we should get a green of 
the same hue as the emerald-green, but not so intense, so that 
in order to match it we should have to mix the green with white 
to dilute it, and with black to make it darker. But it is not in 
this way that we have to arrange the colours, for our blue and 
yellow produce a pinkish tint, and never a green, so that we 
must add green to the combination of blue and yellow, to pro- 
duce a neutral tint, identical with a mixture of white and black. 
Blue, green, and yellow must therefore be combined on the 
large discs, and stand on one side of the equation, and black and 
white, on the small discs, must stand on the other side. In 
order to facilitate calculations, the colours are always put down 
in the same order; but those belonging to the small dises are 
marked negative. Thus, instead of writing 
54U + 14G +32Y =32W + 68B, 
we write =» 4. 54U + 14G —32 W—68B + 32Y=0. 
The sum of all the positive terms of such an equation is 100, 
being the whole number of divisions in the circle. The sum of 
the negative terms is also 100. 
The second equation consists of all the colours except blue ; 
and in this way we obtain six different combinations of five 
colours. 
Each of these combinations was formed by the unassisted 
judgment of my eye, on six different occasions, so that there are 
el ay independent observations of equations between five 
eolours. 
