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IV. On the Theory of Compoimd Colours, and the Belations of the Colours of the 
Spectrum. By J. Cleek Maxwell, M.A., Professor of Natural Philosophy in 
Marischal College and University of Aberdeen. Communicated by Prof essor Stokes, 
Sec. B.S. 
Eeceived January 5, — E-ead March 22, 1860. 
§ I. Introduction. 
Accoedixg to Newton’s analysis of light*, every colour in nature is produced by the 
mixture, in various proportions, of the different kinds of light into which white light is 
divided by refraction. By means of a prism we may analyse any coloured light, and 
determine the proportions in which the different homogeneous rays enter into it ; and 
by means of a lens we may recombine these rays, and reproduce the original coloured 
light. 
Newton has also shownf how to combine the different rays of the spectrum so as to 
form a single beam of light, and how to alter the proportions of the different colours so 
as to exhibit the result of combining them in any arbitrary manner. 
The number of different kinds of homogeneous light being infinite, and the propor- 
tion in which each may be combined being also variable indefinitely, the results of such 
combinations could not be appreciated by the eye, unless the chromatic effect of every 
mixture, however complicated, could be expressed in some simpler form. Colours, as 
seen by the human eye of the normal type, can all be reduced to a few classes, and 
expressed by a few well-known names; and even those colours which have different 
names have ob\ious relations among themselves. Every colour, except purple, is similar 
to some colour of the spectrum J, although less intense; and all purples may be com- 
pounded of blue and red, and diluted with white to any required tint. Brown colours, 
which at first sight seem different, are merely red, orange or yellow of feeble intensity, 
more or less diluted with white. 
It appears therefore that the result of any mixture of colours, however complicated, 
may be defined by its relation to a certain small number of well-known colours. 
Having selected our standard colours, and determmed the relations of a given colour to 
these, we have defined that colour completely as to its appearance. Any colour which 
has the same relation to the standard colours, will be identical in appearance, though 
its optical constitution, as revealed by the prism, may be very different. 
* Optics, book i. part 2. prop. 7. 
t Lectiones Opticse, part 2. § 1. p. 100 to 105 ; and Optics, book i. part 2. prop. 11. 
X Optics, book i. part 2. prop. 4. 
