1861.] on the Theory of Three Primary Colours, 371 



limits of intensity and purity. For instance, if we take carmine (red), 

 chrome yellow, and ultramarine (blue), we get by mixing the carmine 

 and the chrome, all varieties of orange, passing through scarlet to 

 crimson on the one side, and to yellow on the other ; by mixing 

 chrome and ultramarine we get all hues of green ; and by mixing 

 ultramarine with carmine, we get all hues of purple, from violet to 

 mauve and crimson. Now these are all the strong colours that we 

 ever see or can imagine : all others are like these, only less pure in 

 tint. Our three colours can be mixed so as to form a neutral grey ; 

 and if this grey be mixed with any of the hues produced by mixing two 

 colours only, all the tints of that hue will be exhibited, from the pure 

 colour to neutral grey. If we could assume that the colour of a mix- 

 ture of different kinds of paint is a true mixture of the colours of the 

 pigments, and in the same proportion, then an analysis of colour might 

 be made with the same ease as a chemical analysis of a mixture of sub- 

 stances. 



The colour of a mixture of pigments, however, is often very 

 different from a true mixture of the colours of the pure pigments. It 

 is found to depend on the size of the particles, a finely ground pigment 

 producing more effect than one coarsely ground. It has also been 

 shown by Professor Helmholtz, that when light falls on a mixture of 

 pigments, part of it is acted on by one pigment only, and part of it by 

 another ; while a third portion is acted on by both pigments in succession 

 before it is sent back to the eye. The two parts reflected directly from 

 the pure pigments enter the eye together, and form a true mixture of 

 colours ; but the third portion, which has suffered absorption from 

 both pigments, is often so considerable as to give its own character 

 to the resulting tint. This is the explanation of the green tint pro- 

 duced by mixing most blue and yellow pigments. 



In studying the mixture of colours, we must avoid these sources of 

 error, either by mixing the rays of light themselves, or by combining 

 the impressions of colours within the eye by the rotation of coloured 

 papers on a disc. 



The speaker then stated what the opticians had discovered about 

 colour. White light, according to Newton, consists of a great number 

 of different kinds of coloured light which can be separated by a prism. 

 Newton divided these into seven classes, but we now recognize many 

 thousand distinct kinds of light in the spectrum, none of which can be 

 shown to be a compound of more elementary rays. If we accept the 

 theory that light is an undulation, then, as there are undulations of 

 every different period from the one end of the spectrum to the other, 

 there are an infinite number of possible kinds of light, no one of 

 which can be regarded as compounded of any others. 



Physical optics does not lead us to any theory of three primary 

 colours, but leaves us in possession of an infinite number of pure rays 

 with an infinitely more infinite number of compound beams of light, 

 each containing any proportions of any number of the pure rays. 



These beams of light, passing through the transparent parts of the 



2(2 



