MODERN OPTICS AND PAINTING. 



575 



the changes from greenish blue to violet blue, by altering the inten- 

 sity of the original components (Fig. 3). 



It is easy for us now to understand why, in what I some time ago 

 called our fundamental experiment, yellow and blue light, when min- 

 gled, gave not green, but white light ; the yellow light stimulated into 

 action the red and green nerves, the blue light the green and violet 

 ones ; thus, all three sets of nerves being called into play, the result 

 was of course the sensation of white. 



Fig. 3. 





GREEN 





BLUE 





VI 



OLET 





As it will be desirable hereafter to mingle light by the method 

 of revolving disks, it may be well at this point to repeat our funda- 

 mental experiment after this fashion, so as to be assured of the cor- 

 rectness of this mode of experimenting. I have placed in front of 

 the lantern a small circular card-board disk, provided with openings 

 over which are fastened pieces of yellow and blue glass (Fig. 4) ; its 



Fig. 4. 



magnified image now covers pretty much the whole screen, and, on 

 causing it to revolve, the colors as you see vanish, and we have in 

 their place a broad circular band of white light (Fig. 5). With a con- 

 cave mirror, I throw beside it on the screen a direct beam of white 

 light from the lantern, and, if there is any difference, it is in the 

 light from the disk being a little whiter than that of the lantern. 

 The method with revolving disks gives, then, the same result with 

 the more direct one formerly applied, and we can now very conven- 

 iently use it for a final test of the new and old theories. Here is 



