THE THEOET OF COMPOUND COLOUES. 
61 
Accounts of experiments by myself on the mixture of artificial colours by rapid rota- 
tion, may be found in the Transactions of the E-oyal Society of Edinburgh, vol. xxi. 
pt. 2 (1855); in an appendix to Professor George Wilson’s work on Colour-Blindness; 
in the Eeport of the British Association for 1856, p. 12; and in the Philosophical Maga- 
zine, July 1857, p. 40. These experiments show that, for the normal eye, there are 
three, and only three, elements of colom-, and that in the colour-blind one of these is 
absent. They also prove that chromatic observations may be made, both by normal 
and abnormal eyes, with such accuracy, as to warrant the employment of the results in 
the calculation of colour-equations, and in laying down colour-diagrams by Newton’s 
rule. 
The first instrument which 1 made (in 1852) to examine the mixtures of the colours 
of the spectrum was similar to that which I now use, but smaller, and it had no con- 
stant light for a term of comparison. The second was 6|- feet long, made in 1855, and 
showed two combinations of colour side by side. 1 have now succeeded in making the 
mixture much more perfect, and the comparisons more exact, by using white refiected 
light, instead of the second compound colour. An apparatus in which the light passes 
through the prisms, and is refiected back again in nearly the same path by a concave 
mirror, -was shown by me to the British Association in 1856. It has the advantage of 
being portable, and need not be more than half the length of the other, in order to pro- 
duce a spectram of equal length. 1 am so well satisfied with the working of this form 
of the instrument, that 1 intend to make use of it in obtaining equations from a greater 
variety of observers than 1 could meet with when 1 was obliged to use the more bulky 
instrument. It is difficult at first to get the observer to believe that the compound light 
can ever be so adjusted as to appear to his eyes identical with the white light in contact 
with it. He has to learn what adjustments are necessary to produce the requisite 
alteration under all circumstances, and he must never be satisfied till the two parts of 
the field are identical in colour and illumination. To do this thoroughly, implies not 
merely good eyes, but a power of judging as to the exact nature of the difference 
betw^een two veiy pale and nearly identical tints, whether they differ in the amount of 
red, green, or blue, or in brightness of illumination. 
In the folloAvlng paper I shall first lay down the mathematical theory of Newton’s 
diagram, with its relation to Young’s theory of the colour-sensation. I shall then 
describe the experimental method of mixing the colours of the spectrum, and deter- 
mining the wave-lengths of the colours mixed. The results of my experiments will then 
be given, and the chromatic relations of the spectrum exhibited in a system of colour- 
equations, in Newton’s diagram, and in three curves of intensity, as in Brewster’s dia- 
gram. The differences between the results of two observers will then be discussed, 
showing on what they depend, and in what way such differences may affect the vision 
of persons otheixvise free from defects of sight. 
MDCCCLX. 
K 
