ioi2 THE SENSES 



of retina on which the image is formed. The physical condition, 

 then, of our sensations of the prismatic colours is, that rays of 

 approximately the same wave-length should fall unmixed with 

 other rays upon the retinal elements. Rays of a wave-length of 

 760 fift* to 650^ give the sensation of red; from 650 ftp to 590 ^, 

 the sensation of orange; from 430 ^ to 400 (JL^JL, the sensation of 

 violet, and so on. When rays of all these wave-lengths fall together, 

 in the proportion in which they are present in sunlight, upon the 

 same part of the retina, the resultant physiological effect is very 

 different ; we are no longer able to distinguish red, blue, green, etc. ; 

 we receive the single sensation of white light. The sensation is a 

 simple one; in consciousness we have no hint that it has a multiple 

 physical cause. 



But we find further that it is not necessary for the sensation of 

 white light that waves of every length present in the solar spectrum 

 should be mixed. If rays of wave-lengths 675 ftp (which acting 

 alone produce the sensation of red) be mixed in certain proportions 

 i.e., be allowed to fall on the same part of the retina with rays 

 of wave-length 496 pp (which give the sensation of bluish-green), 

 the resultant sensation is also that of white light. And an in- 

 definite number of sets can be combined, two and two, so as to give 

 the same sensation of white. Such colours are called comple- 

 mentary. The following are pairs of complementary colours: 



Red and bluish-green. Yellow and ultramarine-blue. 



Orange and cyan-blue, f Greenish-yellow and violet. 



The green of the spectrum has no simple complementary colour; 

 purple, a colour not present in the spectrum, but obtained by 

 mixing light from the two spectral extremes i.e., by mixing red 

 and violet may be considered complementary to it. Suppose now 

 that one of a pair of complementary colours is added to the other 

 in greater intensity than is required to give white, the resultant 

 sensation is a colour which has a certain amount of resemblance both 

 to white and to the colour present in excess. Thus, if the two 

 colours are orange and blue, and the blue is present in greater in- 

 tensity than is necessary to give white, the resultant colour is a 

 whitish or pale blue, or, to use the technical phrase, an unsaturated 

 blue. The more nearly the intensity of the blue rays in the mixed 

 light approaches the proportion necessary to give white, the less 

 saturated is the resultant colour; the greater the excess of blue, 

 the more nearly does the resultant sensation approach that of the 

 saturated blue of the spectrum. But any non-saturated spectral 

 colour produced by the mixture of two complementary colours may 

 be equally well produced by the mixture of the corresponding 

 spectral colour with a certain quantity of ordinary white light. 



* (Ap is a symbol representing one-millionth of a millimetre, 

 t Cyan-blue is a greenish-blue. 



