Admit the sunbeam through a slit one inch long and one-twentieth
of an inch wide. Pass it through a prism. Either purchase one or
make it of three plain pieces of glass one and a half inch wide
by six inches long, fastened together in triangular shape--fasten
the edges with hot wax and fill it with water; then on a screen
or wall you will have the colors of the rainbow, not merely seven
but seventy, if your eyes are sharp enough.
Take a bit of red paper that matches the red color of the spectrum.
Move it along the line of colors toward the violet. In the orange
it is dark, in the yellow darker, in the green and all beyond,
black. That is because there are no more red rays to be reflected
by it. So a green object is true to its color only in the green
rays, and black elsewhere. All these colors may be recombined by
a second prism into white light.
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III.