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.
[Illustration]