90 COMPLEMENTARY COLOURS. [BOOK in. 



may be taken as characteristic complementary colours, the respec- 

 tive wave-lengths being given : 



Red, X 656, Blue Green, \ 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 on 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 middle 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 sensation which we denote as 

 a particular kind of red ; they do this however, not by the simple 

 and exclusive stimulation of a particular 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 excited 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. 



760. 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 have no 

 choice. If however we are allowed three colours instead of two, 

 we have a much greater range. If we take any three colours, 

 provided only that they lie a certain distance apart along the 

 spectrum, we can produce white by mixing them in certain 

 proportions. If we take any red, green and blue, we can by ad- 

 justing the amount of each, that is the intensity of each, produce 

 white. 



We may go further than this. By adjusting the amounts of 

 each of the three colours we can reproduce all the colours of the 



