Pure Simple Colors Things as they Seem
To the eye white light appears a simple, single color. It reveals its compound nature to us only when passed through a prism, when it shows itself to be compounded of an infinite number of colors which Sir Isaac Newton grouped in seven divisions: violet, indigo, blue, green, yellow, orange, and red.
We naturally ask ourselves whether these colors which compose white light are themselves in turn compound? To answer that question, let us very carefully insert a second prism in the path of the rays which issue from the first prism, carefully barring out the remaining six kinds of rays. If the red light is compound, it will be broken up into its constituent parts and will form a typical spectrum of its own, just as white light did after its passage through a prism. But the red rays pass through the second prism, are refracted, and bent from this course, and no new colors appear, no new spectrum is formed. Evidently a ray of spectrum red is a simple color, not a compound color.
If a similar experiment is made with the remaining spectrum rays, the result is always the same: the individual spectrum colors remain simple, pure colors. The individual spectrum colors are groups of simple, pure colors.