the changes from greenish blue to violet blue, by altering the intensity 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 mingled, 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.
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 fundamental experiment after this fashion, so as to be assured of the correctness 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
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 concave 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 conveniently use it for a final test of the new and old theories. Here is