162 Prof. J. D. Forbes on the Classification of Colours. 



Lambert, were white, ochre-yellow*, red and black; and he 

 supposes that a bluish tint may have been obtained by diluting 

 the black with white. Leonardo da Vinci, himself a painter 

 of the first order, appears to have had clear ideas on the sub- 

 ject of the formation of compound colours from their simplest 

 elements; and although he reckons six, namely white, yellow, 

 green, blue, red and black, yet it is scarcely to be supposed 

 that he was not aware that green could be compounded of 

 yellow and blue, and therefore we may probably admit that 

 he regarded blue, red and yellow as the primary colours, to 

 be mixed with white or black according to the degree of sha- 

 dow in which they are to be represented. 



Waller, in the Philosophical Transactions for 1686, attempts 

 a classification of colours and pigments, proceeding upon the 

 basis (which ajipears to be there assumed as generally ad- 

 mitted) that red, yellow and blue are the sole primary colours. 

 This paper was originally accompanied by a diagram of hues, 

 which is, however, wanting in the copies which I have ex- 

 amined. 



Newton's discovery of the composition of white light was, 

 of course, an important step in the theory of the composition 

 of colours generally ; although that apparent paradox did not 

 fail to introduce some difficulties into the explanation of the 

 action of pigments, which have not unnaturally affected the 

 views of persons accustomed to regard the subject solely as one 

 of art, and at the same time to complicate it somewhat by the 

 introduction of seven colours in the complete spectrum, red, 

 orange, yellow, green, blue, indigo and violet. Perhaps it is 

 not too presumptuous to say, that but for some peculiar re- 

 spect for the number seven, and more particularly from a 

 fancied analogy between the spaces occupied by the colours 

 and the musical intervals, Newton would not have classed blue 

 and indigo as distinct colours; in which case we may consider 

 that the Newtonian spectrum consists of the three primary 

 colours, red, yellow and blue, and the three secondary, orange, 

 green and purple. Newton was perfectly aware that by com- 

 bining the primary colours, such as yellow and blue together, 

 a green, not distinguishable from that of the spectrum except 

 hy its refrangibilityf, will be formed ; and he also observed 

 the effect of combining three or more coloured rays, which 

 generally tend to a more or less perfect whiteness, though it 

 does not appear that Newton ever actually formed white light 

 by the partial combination of certain rays of the spectrum. 



* SUaceus, the word translated ochre-yellow, is of very doubtful signifi- 

 cation. 



t Optics, Book I. part 2, prop. iv. 



