822 A MANUAL OF PHYSIOLOGY 



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- 

 length 675 TT f^ (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 

 49^TTjW which give the sensation of bluish-green), the re- 

 sultant sensation is also that of white light. And an indefi- 

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

 give the same sensation of white. Such colours are called 

 complementary. The following are pairs of complementary 

 colours : 



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



Orange and cyan-blue.* Greenish-yellow and violet. 



The green of the spectrum has no simple complementary 

 colour ; purple, a mixture of 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 sensa- 

 tion is a colour which has a certain amount of resemblance 

 both to white and to the colour present in excess. Thus, ii 

 the two colours are orange and blue, and the blue is present 

 in greater intensity 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 comple- 

 mentary colours may be equally well produced by the 

 mixture of the corresponding spectral colour with a certain 

 quantity of ordinary white light. And it is found that when 

 two spectral colours which are not complementary are mixed 

 together the resultant is not white, but a colour which may 

 be matched by some spectral colour lying between the two, 

 either without addition or plus a larger or smaller quantity 

 * Cyan -blue is a greenish-blue. 



