1861.] on the Theory of Three Primary Colours. 373 



But we have no direct consciousness of the contrivances of our 

 own bodies, and we never feel any sensation which is not infinitely 

 complex, so that we can never know directly how many sensations are 

 combined when we see a colour. Still less can we isolate one or more 

 sensations by artificial means, so that in general when a ray enters the 

 eye, though it should be one of the pure rays of the spectrum, it may 

 excite more than one of the three sets of nerves, and thus produce a 

 compound sensation. 



The terms simple and compound, therefore, as applied to colour- 

 ensation, have by no means the same meaning as they have when 

 applied to a ray of light. 



The speaker then stated some of the consequences of Young's 

 theory, and described the tests to which he had subjected it : — 



1st. There are three primary colours. 



2nd. Every colour is either a primary colour, or a mixture of 

 primary colours. 



3rd. Four colours may always be arranged in one of two ways. 

 Either one of them is a mixture of the other three, or a mixture of two 

 of them can be found, identical with a mixture of the other two. 



4th. These results may be stated in the form of colour-equations, 

 giving the numerical value of the amount of each colour entering into 

 any mixture. By means of the Colour Top,* such equations can be 

 obtained for coloured papers, and they may be obtained with a degree 

 of accuracy showing that the colour-judgment of the eye may be ren- 

 dered very perfect. 



The speaker had tested in this way more than 100 different pig- 

 ments and mixtures, and had found the results agree with the theory 

 of three primaries in every case. He had also examined all the 

 colours of the spectrum with the same result. 



The experiments with pigments do not indicate what colours are 

 to be considered as primary ; but experiments on the prismatic spec- 

 trum show that all the colours of the spectrum, and therefore all the 

 colours in nature, are equivalent to mixtures of three colours of the spec- 

 trum itself, namely, red, green (near the line E), and blue (near the 

 line G). Yellow was found to be a mixture of red and green. 



The speaker, assuming red, green, and blue as primary colours, then 

 exhibited them on a screen by means of three magic lanterns, before 

 which were placed glass troughs containing respectively sulphocyanide 

 of iron, chloride of copper, and ammoniated copper. 



A triangle was thus illuminated, so that the pure colours appeared 

 at its angles, while the rest of the triangle contained the various mix- 

 tures of the colours as in Young's triangle of colour. 



The graduated intensity of the primary colours in different parts of 



♦ Described in the Trans, of the Royal Society of Edinburgh, Vol. XXL, and 

 in the Phil. Mag. 



