520 M. H. Helmholtz un the Theory of Compound Colours. ■ 



strictly speaking not colours, he mentions four, namely yellow, 

 green, blue, and red. The primitive colours red, yellow, and 

 blue, aften\ards generally recognized, have been made the basis 

 of an experiment of ^Yaller described in the Philosophical Trans- 

 actions for 1686, hence before the time of Newton's investigations 

 on the decomposition of white light by the prism, and when no 

 other method of compounding colours save that of mixing the 

 colouring matters was known. In later attempts to classify the 

 natural colours according to their composition by the three pri- 

 mitive ones above mentioned, Castell, the astronomer ]\Iayer, 

 Lambert, Hay, and Forbes*, have all taken as basis of their en- 

 deavours the mixing of the colouring matters. As representants 

 of the primitive colours from which all others might be formed, 

 Mayer made use of cinnabar, kings-yellow, and mountain-blue ; 

 Lambert, carmine, gamboge, and Prussian- blue, which give purer 

 mixtures ; and Hay, whose skill in the choice and use of colours 

 for this purpose is praised by Forbes, carmine, chrome-yellow, 

 and French ultramarine. 



Some physicists attemjited to demonstrate the objective exist- 

 ence of the three primitive colours. Mayer Avas the first to 

 give utterance to the \\cvf that the three primitive colours might 

 cm-respond to three different kinds of light, red, yellow, and blue, 

 each of which furnished rays of all degrees of refrangibility. 

 According to this, at every point of the spectrum red, yellow and 

 blue rays are mixed together, which however do not differ in re- 

 frangibility, and therefore cannot be separated by the prism. 



At the red end of the spectrum the red light was supposed to 

 be predominant, at the blue end the blue, in the middle the 

 yellow. The same view was afterwards expressed by Brewster ; 

 and this celebrated physicist imagined that he was able, by ab- 

 sorption in transparent coloured media, actually to separate the 

 diflPerent kinds of light in all parts of the spectrum. 



After his discovery of the composition of white light, Newton 

 assumed the existence of seven principal colours in the spectrum : 

 red, orange, yellow, green, blue, indigo, violet. He chose this 

 nvimber probably because of the analogy which he sought between 

 the colours and the musical intervals of the scale, and this also 

 suggested the divisions of his seven-coloured disk. This accounts 

 for the distinction which he has drawn between blue and indigo- 

 blue. Tliat this distinction has been made in the blue is, in all 

 likelihood, to be referred to the fact that in most prisms the blue 

 portion is comparatively expanded, ai;d the breadth of the bands 



* T. Castell, Farben clavier. Mayer m Got finger gel. Anzeigen 1/58, 

 p. 147. J. II. Lambert, Beschreibung einer Farbenpyramide: Berlin, 17/2. 

 D. R. Hay, Nomenclature of Colours. J. D. Forbes in Phil. Mag. S. 3 . 

 vol. xx.\iv. p. 161. 



