[Lecture at the Royal Institution of Great Britain. May 17, 1861.] 



XXII. On the Theory of Three Primary Colours. 



THE speaker commenced by shewing that our power of vision depends 

 entirely on our being able to distinguish the intensity and quality of colours. 

 The forms of visible objects are indicated to us only by differences in colour 

 or brightness between them and surrounding objects. To classify and arrange 

 these colours, to ascertain the physical conditions on which the differences of 

 coloured rays depend, and to trace, as far as we are able, the physiological 

 process by which these different rays excite in us various sensations of colour, 

 we must avail ourselves of the united experience of painters, opticians, and 

 physiologists. The speaker then proceeded to state the results obtained by these 

 three classes of inquirers, to explain their apparent inconsistency by means of 

 Young's Theory of Primary Colours, and to describe the tests to which he had 

 subjected that theory. 



Painters have studied the relations of colours, in order to imitate them by 

 means of pigments. As there are only a limited number of coloured substances 

 adapted for painting, while the number of tints in nature is infinite, painters 

 are obliged to produce the tints they require by mixing their pigments in 

 proper proportions. This leads them to regard these tints as actually com- 

 pounded of other colours, corresponding to the pure pigments in the mixture. 

 It is found, that by using three pigments only, we can produce all colours 

 lying within certain 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 



