VISUAL SENSATIONS. 771 



ture 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 sensa- 

 tions 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 color of that ray, but that th ray 

 gives rise either to a complex impulse which becomes converted into a com- 

 plex sensation, or to a simple impulse which eventually develops into a 

 mixed or complex sensation, into the composition of which in each case 

 other orange tints and shades of red and yellow enter. 



663. 2. When certain colors are mixed together in pairs in certain 

 definite proportions, the result is white. These colors are : 



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



Orange (nearC), and Blue (between F and G) ; 



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



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



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

 added the peculiar non-prismatic color purple, which with green also 

 gives white. 



3. If we select arbitrarily any three colors 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 colors can be taken in such pro- 

 portions as with a proper addition, if necessary, of white to produce the 

 sensations of all other colors. 2 That is to say* given three standard sen- 

 sations, all the other sensations may be gained by the proper mixture of 

 these. 



664. It is obvious from the foregoing that our real color sensations are 

 much fewer in number than those which we appear to have when we look on 

 the colors of the spectrum or of nature ; that rays of light awake in us cer- 

 tain simple sensations, which mixed in various proportions reproduce all our 

 sensations. And the question arises, What is the nature or what are the 

 characters of these simple sensations ? 



When we examine our own sensations of light we find that certain of 

 these seem to be quite distinct in nature from each other, so that each is 

 something sui generis, whereas we easily recognize all other sensations as 

 various mixtures of these. Thus red and yellow are to us quite distinct ; 

 we do not recognize anything common to the two ; but orange is obviously 

 a mixture of red and yellow. The sensations caused by different kinds of 



1 These letters refer to Frauenhofer's lines. 



2 A few highly saturated colors cannot be so reproduced, but a mixture of any one of 

 them with white can. We may, perhaps, therefore speak of these saturated colors as 

 being reproduced by a proper combination of the three arbitrarily selected colors, with 

 the subtraction of white. 



