172 Prof. J. D. Forbes ow the Classification qfColojirs. 



size and in the number oF elements, as the standard colours 

 approached white on the one hand and black on the other ; 

 forming thus a double pyramid, whose common base was 

 Mayer's triangle, the colours vanishing into white at one apex 

 and into bhick at the other*. A triangular pyramid or tetra- 

 hedron of 13 elements in the side would contain 4-55 elements, 

 and the double pyramid or hexahedron 910 elements. 



Other writers have attempted to adopt primary colours dif- 

 ferent from red, yellow and blue; and with this subject has 

 been mixed up the inquiry into the actual composition of the 

 solar spectrum, which though not immediately connected with 

 it, may be mentioned in passing. Mayer maintained, not merely 

 that all colours whatever may be formed by the combination of 

 red, yellow and blue, but that in reality these colours alone 

 exist in the solar lightf) which he inferred by looking through 

 a prism at a black spot on a white ground, an essentially 

 faulty mode of operating. But later. Dr. Wollaston came to 

 the conclusion that the solar spectrum is composed of four 

 colours only, namely red, green, blue and violet, without any 

 gradations in the quality of the colour. Dr. Young, finding ex- 

 perimentally that " the perfect sensations of yellow and of blue 

 are produced respectively by mixtures of red and green, and 

 of green and violet light J," assumes that the primary colours 

 are red, green and violet ; a singular opinion, which appears 

 to rest on no particular evidence further than the disjunction 

 of the red and violet rays at the two ends of the spectrum, and 

 which has met with no support any more than that of Abbe 

 Nollet, who maintained the primary qualities of orange, green 

 and purple. In truth no synthetical experiment can give any 

 sure countenance to one or other of these views ; for the fact 

 that red and green combined in certain proportions produce 

 yellow, admits of equally sound interpretation by supposing 

 that the green, being a compound of yellow and blue, the 

 whole of the blue and a part of the yellow combine with the 

 red to produce a perfect white, which then dilutes the out- 

 standing portion of the yellow; and in like manner a perfect 

 purple mixed with perfect green must make a perfect blue 

 diluted with a perfect white. Analysis, however, where pos- 

 sible, must lead to more conclusive results; and Sir David 

 Brewster considers that the orange, green, and purple of the 

 spectrum are really composed of two, if not three colours 



* In the coloured plate accompanying Lambert's work we find only the 

 pyramid of colour dihited with white, which he seems to have considered 

 sufficient in practice, § 39. In reality, however, the shades or mixtures 

 with black are indispensable components of such a system. 



+ Mayer, Gottinmschen Anzeigen, quoted by Lambert, p. 30. 



t Lecture XXXVII. 



