[, 4. 4 
IV. Account of Experiments on the Perception of Colour. By 
J. C. Maxwett, B.A., Professor of Natural Philosophy, 
Marischal College, Aberdeen. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
ft Nate experiments which I intend to describe were undertaken 
in order to render more perfect the quantitative proof of 
the theory of three primary colours. According to that theory, 
every sensation of colour in a perfect human eye is distinguished 
by three, and only three, elementary qualities, so that in mathe- 
matical language the quality of a colour may be expressed as a 
function of three independent variables. There is very little 
evidence at present for deciding the precise tints of the true 
primaries. I have ascertained that a certain red is the sensation 
wanting in colour-blind eyes, but the mathematical theory relates 
to the number, not to the nature of the primaries. If, with Sir 
David Brewster, we assume red, blue, and yellow to be the primary 
colours, this amounts to saying that every conceivable tint may 
be produced by adding together so much red, so much yellow, 
and so much blue. This is perhaps the best method of forming a 
provisional notion of the theory. It is evident that if any colour 
could be found which could not be accurately defined as so 
much of each of the three primaries, the theory would fall to 
the ground. Besides this, the truth of the theory requires that 
every mathematical consequence of assuming every colour to be 
the result of mixture of three primaries should also be true. 
_T have made experiments on upwards of 100 different artifi- 
cial colours, consisting of the pigments used in the arts, and their 
mechanical mixtures. These experiments were made primarily 
to trace the effects of mechanical mixture on various coloured 
powders; but they also afford evidence of the truth of the 
theory, that all these various colours can be referred to three 
primaries. The following experiments relate to the combina- ” 
tions of six well-defined colours only, and I shall describe them 
the more minutely, as I hope to induce those who have good 
ores to subject them to the same trial of skill in distinguishing 
ints. 
The method of performing the experiments is described in 
the Transactions of the Royal Society of Edinburgh, vol. xxi. 
part 2. The colour-top or teetotum which I used may be had . 
of Mr. J. M. Bryson, Edinburgh, or it may be easily extempo- 
nized. Any rotatory apparatus which will keep a dise revolving 
steadily and rapidly in a good light, without noise or disturb- 
ance, and can be easily stopped and shifted, will do as well as 
the contrivance of the spinning-top. 
