164 Prof. J. D. Forbes on the Classificatioji of Colours. 



cannot be compounded out of only txw colours in the spec- 

 trum. This corresponds with Newton's experience, that such 

 a colour (the mixture of two opposites) " shall not be perfectly 

 white, but some faint anonymous colour*." But these expe- 

 riments merit well a careful repetition, which I am not indeed 

 aware that they have ever received ; and it is very probable 

 that Newton never made them with a pure, or even an ap- 

 proximately pure spectrum f. 



But Newton's celebrated experiment of mixing together 

 coloured powders until he obtained a perfectly indefinite gray 

 is most to our present purpose. He describes in the fifteenth 

 experiment of the second part of his first book of Optics, the 

 various dry pigments which he employed, the most effective 

 of which was a mixture of orange, purple, green and blue, 

 which " became of such a gray or pale white as verged to no 

 one of the colours more than to another" (p. 131), which 

 when powerfully illuminated by the sun was an exact match 

 for a pure white paper less perfectly illuminated. The reason 

 why it does not appear absolutely white under ordinary cir- 

 cumstances Newton thus explains: — "All coloured powders 

 do suppress or stop in them a very considerable part of the 

 light by which they are illuminated. For they become co- 

 loured by reflecting the light of their own colours more co- 

 piously and that of all other colours more sparingly, and yet 

 they do not reflect the light of their own colours so copiously 

 as white bodies do." [This he illustrates by illuminating red 

 lead and white paper with the red ray ; the white paper ap- 

 pears the more brilliantly red of the two.] " Therefore by 

 mixing such powders [powders, namely, of various colours,] 

 we are not to expect a strong and full white, such as is that 

 of paper, but some dusky obscure one such as might arise 

 from a mixture of light and darkness, or from white and black, 

 that is, a gray, or dun, or russet-brown, such as are the co- 

 lours of a man's nail, of a mouse, of ashes, of ordinary stones, 

 of mortar, of dust and dirt in the highways and the like." — 

 P. 130. 



Whatever may be thought of Newton's theory of the colours 



• Optics, ed. cit. p. 136. 



t If Newton's circular ti<jfiire was intended (which, however, M. Biot 

 questions) to be divided according to the accurate proportions of the colours 

 in the spectrum, it would be a matter of great difficulty to assign the due 

 proportions to the extreme red and violet ; any variation in this respect 

 would alter the character of the diametrically opposed tints, and make a 

 perfectly white compound of two tints possible, or the reverse. It is hardly 

 necessary to add here, that the perfect white produced by the comple- 

 mentary tints which occur in experiments of depolarization, arise:) from the 

 mixture of colours of a very complex constitution. 



