366 PHYSIOLOGY CHAP. 



ever, there is uo corresponding gradation of visual sensation. 

 Our eye is only capable of distinguishing a limited number of 

 colours and shades of colour (according to Schenck, about 200), 

 each of which arises from a more or less extensive region of 

 the spectrum, corresponding to a greater or less number of 

 simple lights. The generally admitted distinctions of the spectral 

 colours are more or less artificial. Aubert, Wundt, and others, 

 however, distinguish four primary colours known in all the 

 classical languages by specific, simple names (red, yellow, green, 

 blue), as distinct from the intermediate colours which are 

 designated by compound names (orange-yellow, yellow-green, blue- 

 green, violet-blue, etc.). We shall find that Hering adopted this 

 theory and arranged the four primary colours in two pairs of 

 opponent colours (yellow-blue and red-green). 



Ketiual discrimination of shades of colour varies in the 

 different parts of the spectrum. According to Dobrowolski, 

 sensibility is maximal for yellow and blue, minimal for red and 

 green. 



The spectral colours are the purest it is possible to obtain : 

 they are also the most saturated, that is the least diluted with 

 white, when they result from rays of medium intensity. 



It is otherwise with the colours of objects and the dyes or 

 pigments used by dyers and painters, which result from tin- 

 reflection of the rays that give the colour to the object, with 

 simultaneous absorption of all other chromatic rays. The tone 

 of these varies very much according to the smoothness of surface 

 of the object, its transparency and the degree of absorption of the 

 different chromatic rays. They are never saturated like the 

 spectral colours, but always contain a greater or less amount 

 of white. 



Painters happily distinguish between warm colours and cold 

 colours expressions which have a real, and not merely a 

 metaphorical significance. Orange and yellow are warm colours ; 

 blue, indigo, and violet are cold colours ; green is an intermediate 

 shade, warm if it inclines to yellow, cold if it verges on blue. 

 Ked and yellow are the colours of fire and of sunlight ; blue and 

 violet those of the weaker moonlight. Helmholtz observed that 

 on looking through yellow glasses on a dull day the land- 

 scape assumes the aspect of sunshine ; with blue glasses, on the 

 contrary, the finest day presents the appearance of moonlight. 



The compound light of an oil lamp, candle, or gas flame is 

 usually very defective in blue and violet rays, and diffuses a warm 

 tone of colour. The electric arc light, on the contrary, is rich in 

 these rays, and diffuses a cold, bluish tint. 



Newton first synthetised white light, by mixing the different 

 chromatic rays that had been analysed by the prism, and 

 formulated the most universal laws of colour-mixture ; Graumann 



