112 HABIT AND INTELLIGENCE. [chap. 



All colours Except white, all possible colours are colours of the spectrum. 

 e.xeept Black is merely the negation of Light, and grey is only a subdued 

 in the or lowered white.^ Brown tints, which to the eye appear unlike 

 Black"™' ^^y °^ *'^® colours of the spectrum, are " merely red, orange, or 

 Grey. yellow, of feeble intensity, more or less diluted with white." ^ 



Brown. j^^ ^^^ colours of the spectrum not complementary to each other 

 are mixed in the way already described (the brightness of the two 

 Eesult of colours being of equal intensity), the colour produced by their 

 combining combination will be similar in appearance to the colour situated 



two ^ '■ 1 • p 



colours not half-way between the two constituents on the circumference ot 



comple- ^-j^g circle of prismatic colours : and the compound will not 

 mentary, ^ . 



necessarily be similar in appearance to either constituent. 



i.9 to form Thus, red and green form yellow. But such a compound 



compound colour, though it may be chromatically similar to a simple one, 



vLsiblv is of course optically different, and may be decomposed back 



like simple jjj^Q j^g constituents by the prism. The compound colours, 



o])tical]y however, though of the same tint with the simple ones, are in 



different, many cases less saturated, presenting the appearance of being 



diluted with white. 



No dis- It follows from this, that there is no distinction of primary 



tinction of j^^^j secondary colours,^ for every colour may be either primary 



and ' or secondary : primary, because it is found in the spectrum ; and 



second- secondary, because it may be formed by the combination of 



aries in /' '' '' 



any pliy- other colours. 



.sical sense, j^ ^Iso follows by mathematical consequence from the facts 

 stated here, and has been verified by experiment, that if any 

 three colours whatever are so taken from the circumference of 

 the circle of prismatic colours, that the centre of the circle falls 

 within the triangle whereof they are at the angles, either white 



1 Most of wlwt are callcil grey tints, however, probably contain 

 bine. 



2 Professor Clark Maxwell's paper, referred to in a former note. 



3 Sir David Brewster thought he had shown that the colours of the 

 spectrum are not primary, but capable of further decomposition by passing 

 them through coloured media. By this kind of decomposition he thought 

 he proved that there are but three primary colours, namely red, yellow, 

 aiid blue. Helmholtz however has, 1 think, shown satisfactorily that 

 Brewster's results were due to an imperfect method of observing, and that 

 no ray of tlie spectrum is capable of decomposition by passing through any 

 coloured medium. A ray under such circum.stances may be gi-eatly weak- 

 ened, but it preserves its tint uuchauged. (See Helmholtz's paper in the 

 Philosophical Magazine for December 18r>2.) 



