By the seventeenth century glass makers had learned to make gem-shaped pieces that were used in chandeliers. Candlelight was refracted in these pieces. When viewing the candlelight different colors were seen, depending on the angles made by the light, the glass, and the viewer’s eye. Newton, who was intrigued by the colors that were produced by those chandeliers, decided to examine how a piece of glass shaped as a rectangular prism could create a spectrum of colors. In his own words: “In a darkened Room make a hole in the [windowshade] of a window, whose diameter may conveniently be about [one-third] of an inch, to admit a convenient quantity of the Sun’s light: And there place a clear and colourless Prisme, to refract the entering light towards the further part of the Room, which, as I said, will thereby be diffused into an oblong [spectrum].”

To prove that the colors did not come from the prism, Newton expanded the experiment by reversing the procedure and forming white light from the spectrum of colors. He accomplished this by placing a lens in the middle of the spectrum to converge the colors on a second prism in the path of the colors. Sure enough, a beam of white light emerged out of the second prism.