Account of Experiments on the Perception of Colour. 44 
The essential part of the experiment consists in placing several 
dises of coloured paper of the same size, and slit along a radius, 
over one another, so that a portion of each is seen, the rest being 
covered by the other discs. By sliding the discs over each other 
the proportion of each colour may be varied, and by means of 
divisions on a circle on which the dises lie, the proportion of each 
colour may be read off. My circle was divided into 100 parts. 
On the top of this set of discs is placed a smaller set of con- 
centric discs, so that when the whole is in motion round the 
centre, the colour resulting from the mixture of colours of the 
small discs is seen in the middle of that arising from the larger 
discs. It is the object of the experimenter to shift the colours 
till the outer and inner tints appear exactly the same, and then 
to read off the proportions. 
It is easy to deduce from the theory of three primary colours 
what must be the number of discs exposed at one time, and how 
much of each colour must appear. 
Every colour placed on either circle consists of a certain pro- 
portion of each of the primaries, and in order that the outer and 
imner circles may have precisely the same resultant colour in 
every respect, there must be the same amount of each of the 
primary colours in the outer and inner circles. Thus we have 
as many conditions to fulfil as there are primary colours; and 
besides these we have two more, because the whole number of 
divisions in either the outer or the inner circle is 100, so that if 
there are three primary colours there will be five conditions to 
fulfil, and this will require five discs to be disposable, and these 
must be arranged so that three are matched against two, or four 
against one. 
If we take six different colours, we may leave out any one of 
the six, and so form six different combinations of five colours. 
It is plain that these six combinations must be equivalent to 
two equations only, if the theory of three primaries be true. 
The method which I have found most convenient for regis- 
tering the result of an experiment, after an identity of tint has 
been obtained in the inner and outer circles, is the following :— 
Write down the names or symbols of the coloured discs each 
at the top of a column, and underneath write the number of 
degrees of that colour observed, calling it + when the colour is 
in the outer circle, and — when it is in the inner circle; then 
equate the whole to zero. In this way the account of each 
colour is kept in a separate column, and the equations obtained 
are easily combined and reduced, without danger of confound- 
ing the colours of which the quantities have been measured. 
The following experiments were made between the 3rd and 11th 
of September, 1856, about noon of each day, in a room fronting 
