The prism is a triangular solid of glass, and by it the young optician
may decompose a ray of light into its primitive and supplementary
colors, for a ray of light is of a compound nature. By the prism the
ray is divided into its three primitive colors, blue, red, and yellow;
and their four supplementary ones, violet, indigo, green, and orange.
The best way to perform this experiment is to cut a small slit in a
window-shutter, on which the sun shines at some period of the day, and
directly opposite the hole place a prism; a beam of light in passing
through it will then be decomposed, and if let fall upon a sheet of
white paper, or against a white wall, the seven colors of the rainbow
will be observed.