526 COLOUR SENSATIONS. [BOOK in. 



colours are fused together in various combinations, the following 

 remarkable results are brought about. 



1. When red and yellow in certain proportions are mixed 

 together the result is a sensation of orange, quite indistinguishable 

 from the orange of the spectrum itself. Now the latter is produced 

 by rays of certain wave-lengths, whereas the rays of red and of 

 yellow are respectively of quite different wave-lengths. The orange 

 of the spectrum cannot be made up by any mixture of the 

 red and the yellow of the spectrum in the sense that the red 

 and yellow rays can unite together to form rays of the same wave- 

 lengths as the orange rays ; the three things are absolutely different. 

 It is simply the mixed sensation of the red and yellow which is so 

 like the sensation of orange ; the mixture is entirely and absolutely 

 a physiological one. In the same way we may by appropriate 

 mixtures produce the sensations corresponding to other parts of the 

 spectrum. Now we must suppose that rays of different wave- 

 lengths give rise to different sensory impulses, that, for instance, the 

 sensory impulses generated by orange rays are different from those 

 generated by red and by yellow rays. Hence we are led by 

 the fact of mixed sensations being identical with other apparently 

 simple sensations to infer that the sensory impulses which any 

 ray originates are either themselves of a complex character, or in 

 becoming converted into sensations give rise to complex or mixed 

 sensations ; that, for instance, the impulse or sensation which a ray 

 in the middle of the orange gives rise to, is not a simple impulse or 

 sensation answering exclusively to the colour of that ray, but 

 that the ray gives rise either to a complex impulse which becomes 

 converted into a complex sensation, or to a simple impulse which 

 eventually developes into a mixed or complex sensation, into the 

 composition of which in each case other orange tints and shades 

 of red and yellow enter. 



2. When certain colours are mixed together in pairs in certain 

 definite proportions, the result is white. These colours are 



Red (near a) 1 , and Blue-Green (near F), 



Orange (near C), and Blue (between F and G), 



Yellow (near D), and Indigo-Blue (near G), 



Green- Yellow (near E), and Violet (between G and H), 



and are said to be 'complementary' to each other. To these might 

 be added the peculiar non-prismatic colour purple, which with 

 green also gives white. 



3. If we select arbitrarily any three colours corresponding to 

 any three parts of the spectrum sufficiently far apart, say for 

 instance red, green, and blue, we can, by a proper adjustment of 

 the proportions of each, produce white. Further, these three 



1 These letters refer to Frauenhofer's lines. 



