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THE POPULAR SCIENCE MONTHLY. 



than those that produce the sensation of red, but longer than those 

 that give us that of green ; referring to Fig. 1, we see that no espe- 

 cial set of nerves has been provided for this case, but a moment's reflec- 

 tion will suggest that these intermediate waves, according to our the- 

 ory, ought to set into moderate action both the red and green nerves, 

 that the stimulation of the former should predominate as the length 

 of our intermediate waves is made longer ; the green set, on the other 

 hand, coming more into play as it is shortened. This accounts, then, 

 for the mode in which waves of a certain length, or light of a certain 

 kind, gives us the sensation of yellow or orange. The light may be 

 simple, and of only one kind, but it produces a compound sensation, 

 made up of the two simple sensations, red and green. From all this 

 it follows that, on the other hand, if we actually present to the same 

 eye mixtures of red and green light, the sensations of yellow or 

 orange ought, according to our theory, to be produced. This is a 

 matter that we can easily test by experiment. With the same appa- 

 ratus used a moment ago for combining blue and yellow light, I 

 throw upon the screen a large square of red light, and superimpose on 

 it one of green, and, as you see, the result is a fair yellow ; on redu- 

 cing the brightness of the green component, the yellow passes into 

 orange (Fig. 2). I call your attention, in passing, to the circumstance 



Fig. 2. 





BED 







YELLOW 







GREEN 





that, according to the old theory, the result ought not to have been 

 yellow, but rather an approach to white, all the colors, according to 

 its doctrines, being present. Restoring the green squares to their 

 original brightness, and reducing the intensity of the red light, we 

 easily obtain a greenish yellow, completing thus this series of tints. 



And now to account for the blue : pure blue light has a wave- 

 length intermediate between that of green and violet light, and hence 

 sets both the green and violet nerves into action, and, though the 

 light itself may be simple, it produces a compound sensation which 

 we call blue. Corresponding to this, I ought to be able to reproduce 

 on the screen blue light by mixing together green and violet light. 

 The experiment is now arranged, and, as you see, we actually do ob- 

 tain a quite good blue in this way, and can cause it to run through all 



