THE THEOEY OF COMPOUm) COLOIJES. 
59 
for practice, though not mathematically accurate.” He gives no reasons for the ditferent 
parts of his rule, but we shall find that his method of finding the centre of gravity of 
the component colours is completely confirmed by my observations, and that it involves 
mathematically the theory of three elements of colour; but that the disposition of the 
colours on the circumference of a circle was only a provisional arrangement, and that 
the true relations of the colours of the spectrum can only be determined by direct 
observation. 
Youxg* appears to have originated the theory, that the three elements of colour are 
determined as much by the constitution of the sense of sight as by anything external to 
us. He conceives thai tlmee difierent sensations may be excited by light, but that the 
proportion in which each of the three is excited depends on the nature of the light. 
He conjectures that these primary sensations correspond to red, green, and violet. A 
blue ray, .for example, though homogeneous in itself, he conceives capable of exciting 
both the green and the Holet sensation, and therefore he would call blue a compound 
colour, though the colour of a simple kind of light. The quality of any colour depends, 
according to this theory, on the ratios of the intensities of the three sensations wliich it 
excites, and its hrightness depends on the sum of these three intensities. 
Sh- David Beewstee, in his paper entitled “ On a New Analysis of Solar Light, indi- 
cating three Primary Coloiu’s, foiming Coincident Spectra of equal length f,” regards the 
actual colours of the spectrum as arising from the intermixture, in various proportions, 
of three primary kinds of light, red, yellow, and blue, each of vrhich is variable in inten- 
sity, but uniform in colour, from one end of the spectrum to the other ; so that every 
coloiu’ in the spectrum is really compound, and might be shown to be so if we had the 
means of separating its elements. 
Sir David Beewstee, in his researches, employed coloured media, which, according to 
him, absorb the three elements of a single prismatic colour in difierent degrees, and 
change their proportions, so as to alter the colour of the light, without altering its 
refrangibility. 
In this paper I shall not enter into the very important questions afiecting the phy- 
sical theory of hght, which can only be settled by a careful inquiry into the phenomena 
of absorption. The physiological facts, that we have a threefold sensation of colour, 
and that the three elements of this sensation are afiected in difierent proportions by light 
of difierent refrangibilities, are equally true, whether we adopt the physical theory that 
there are three kinds of light corresponding to these three colour-sensations, or whether 
we regard light of definite refrangibility as an undulation of known length, and there- 
fore variable only in intensity, but capable of producing difierent chemical actions on 
difierent substances, of being absorbed in difierent degrees by difierent media, and of 
exciting in difierent degrees the three chfierent colour-sensations of the human eye. 
* Torxo’s Lectures on IS'atural Pliilosopliy, Kellaxd’s Edition, p. 345, or Quarto, 1807, vol. i. p. 441 
see also Young in Pliilosopliical Transactions, 1801, or Works in Quarto, vol. ii. p. 617. 
t Transactions of tlie Eoyal Society of Edinburgh, vol. sii. p. 123. 
