AND MODE11N PHYSICS. 95 



letter to Dr. G. Wilson, printed in the Transactions of 

 the Royal Society of Arts for 1855 ; but he had been 

 mixing colours by means of his top for some little time 

 previously, and the results of these experiments are 

 given in a paper entitled "Experiments on Colour," 

 communicated to the Royal Society of Edinburgh 

 by Dr. Gregory, and printed in their Transactions, 

 vol. XXL 



In the paper on " The Theory of Compound 

 Colours," printed in the Philosophical Traasactions 

 for I860, Maxwell gives a history of the theory as 

 it was known to him. 



Ho points out first the distinction between the 

 optical properties and the chromatic properties of a 

 beam of light. "The optical properties are those 

 which have reference to its origin and propagation 

 through media until it falls on the sensitive organ of 

 vision ; " they depend on the periods and amplitudes 

 of the ether vibrations which compose the beam. 

 "The chromatic properties are those which have 

 reference to its i>ower of exciting certain sensations of 

 colour perceived through the organ of vision." It is 

 possible for two beams to be optically very different 

 and chromatically alike. The converse is not true; 

 two beams which are optically alike are also chroma- 

 tically alike. 



The foundation of the theory of compound colours 

 was laid by Xewton. lie first shewed that "by the 

 mixture of homogeneal light colours may be pro- 

 duced which are like to the colours of homogeneal 

 light as to the appearance of colour, but not as to tho 

 immutability of colour and constitution of light." Two 



