YELLOW AND BLUE DO NOT MAKE GREEN.
With an electric or lime-light throw a disc of blue light and of yellow light upon a screen, and cause them to overlap each other. Where they overlap the space on the screen will be, not green, but a pure white. If you then place a rod or pencil near the two sources of light, so that two shadows of it shall fall on the white space where the discs overlap, one shadow will be of brilliant blue colour, and the other deep yellow. Mixed blue and yellow lights, therefore, do not make green. Mixed blue and yellow paints make green, because between them they absorb nearly all the rays of the spectrum except green, so green is the only colour which escapes from the mixture.
A very curious practical illustration of this may be given. Everybody has a yellow spot, more or less marked, on the retina of the eye; and this yellow spot absorbs some of the greenish-yellow rays of the spectrum. If you throw on the screen a circle of light, coloured by passing the rays from the electric lamp through chloride of chromium, the disc will then consist chiefly of red rays mixed with the rays which are absorbed by the yellow spot. If the observers in the darkened theatre look at the disc and wink slowly, they will see, with more or less distinctness, an illusion like red clouds floating over the disc, in consequence of most of the rays other than the red being absorbed by the yellow spots in their eyes.
