THE THEORY OF COLOURS IN RELATION TO COLOUR-BLINDNESS. 125 



to practice ; to Helmholtz for a rigorous examination of the facts on which it 

 rests; and to Prof. Grassrnan (in the Phil. Mag. for 1852), for an admirable 

 theoretical exposition of the subject. The colours given in Hay's Nomenclature 

 of Colours are illustrations of a similar theory applied to mixtures of pigments, 

 but the results are often different from those in which the colours are combined 

 by the eye alone. I hope soon to have results with pigments compared with 

 those given by the prismatic spectrum, and then, perhaps, some more definite 

 results may be obtained. Yours truly, 



J. C. MAXWELL. 



EDINBURGH, 4th Jan. 1855. 



