CHAP, in.] SIGHT. 897 



other. In order to get a complete white, that is a white free 

 from all colour, a certain proportion between the relative 

 amounts of red and green light, that is to say between the 

 intensities of the two sensations, must be observed. And it 

 will be understood that the white thus produced by two small 

 parts of the spectrum is not equal in intensity to the white 

 which would be produced by the combined effect of the whole 

 of the same spectrum. The following may be taken as char- 

 acteristic complementary colours, the respective wave-lengths 

 being given : 



Red, X 656, Blue Green, X 492, 



Orange, X 608, Blue, X 490, 



Gold Yellow, X 574, Blue, X 482, 



Yellow, X 564, Indigo-blue, X 462, 



Greenish Yellow, X 564, Violet, X 433. 



It will be understood that the above are not the only comple- 

 mentary colours ; as we pass from the red end of the spectrum 

 towards the green, each successive part of the spectrum has its 

 complementary part on the other, blue side of the spectrum, 

 each wave-length on the red side has its complementary wave- 

 length on the blue side. When we reach the greenish yellow 

 at X 564, the complementary colour is 011 the very margin of 

 the violet end of the visible spectrum. But we may go, so to 

 speak, outside the spectrum, for the green of the spectrum has 

 for its complementary colour, purple. Or, to put it in another 

 way, while each end of the spectrum has its complementary 

 colour at the other end, the complementary colour of the mid- 

 dle of the spectrum is a combination of the two ends. 



The bearing of these facts on the theory of primary colour 

 sensations is obvious. Two complementary colours excite 

 between them all the primary sensations which are excited by 

 white light, though not to the same intensity. Rays of the 

 wave-length X 656 falling on the retina give rise to the sensa- 

 tion which we denote as a particular kind of red ; they do this 

 however, not by the simple and exclusive stimulation of a par- 

 ticular red sensation, but by exciting all the primary sensations 

 which are not excited by the wave-length X 492. Conversely 

 rays of the wave-length X 492, produce the sensation of blue 

 green by exciting all the primary sensations which are not ex- 

 cited by X 656. Similarly complex is the effect of other wave- 

 lengths. We may roughly describe each of two complementary 

 wave-lengths as stirring up about half the whole of the primary 

 sensations which can be excited by rays of all wave-lengths. 



563. To produce white out of two colours, out of two parts 

 of the spectrum, we are limited to certain pairs ; if we take 

 one colour, we are limited to one other colour, to its pair ; we 



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