446 OX THE TllEORY OF THREE PRIMARY COLOURS. 



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 mixture 

 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 substances. 



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 shewn 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 pig- 

 ments 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 produced 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 shewn 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 propor- 

 tions of any number of the pure rays. 



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



