368 



PHYSIOLOGY 



CHAP. 



pairs of spectral colours, but that each ray of a given wave-length 

 has as its complement another ray of the spectrum. Accord- 

 ingly there is an infinite series of pairs of complementary 

 colour-rays, e.g. 



Red has as its complement . . . blue-green. 



Orange has as its complement . . .. blue. 



Yellow has as its complement . . . indigo. 



Yellow-green has as its complement . . violet. 



Green alone has no complementary colour in the spectrum, 

 but its chromatic quality can be neutralised by a mixture of red and 



FIG. 175. A, Maxwell's colour-discs, which can be 

 rotated by clockwork ; B, a disc of coloured paper, 

 with a slit which allows the partial superposition 

 of other discs of different colours, so as to obtain 

 an area with variable, coloured sectors ; C, metal 

 disc surrounded by a graduated circle which en- 

 ables the angular dimensions of each coloured 

 sector to be read ; the superposed coloured discs 

 (red, green, blue) are fixed at the centre by a 

 screw, with two smaller discs (white and black) to 

 produce the different gradations of grey. 



violet. As the colour purple is obtained by mixing the two ends 

 of the spectrum, it follows that the complementary colour of green 

 is purple. Young and Helmholtz regarded red, green, and violet 

 that is, the extremes and the central colour of the spectrum 

 as the fundamental colours and all the other spectral colours as 

 intermediate. On this they based their theory of colour-vision 

 (infra). 



It follows that white light can be obtained not only by the 

 simultaneous action of all the visible rays of sunlight, but also 

 by the admixture of an indefinite number of pairs of simple 

 colours with a definite difference of wave-length. 



Our eye cannot distinguish between the objectively different 



