[From the Philosophical Trantaotioru, MDCCCLX.] 



XXI. On the Theory of Compound Colours, and the Relations of the Colours 

 of the Spectrum. Communicated by Professor STOKES, Sec. RS. 



(Received January 5, Read March 22, 1860.) 



I. Introduction. 



ACCORDING to Newton's analysis of light*, every colour in nature is pro- 

 duced 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 shewnt 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 

 proportion 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 obvious relations among them- 

 selves. Every colour, except purple, is similar to some colour of the spectrum \, 



* Optici, Book L Part 2, Prop. 7. 



t Lectioneg Opticce, Part 2, 1, pp. 100 to 105; and Optics, Book i. Part 2, Prop. 11. 



I Optict, Book i. Part 2, Prop. 4. 



