Newton's Experimentum Crucis was Newton's proof that white light was composed of colored light rather that different-colored light resulted from differing speed through a medium. In a number of versions of this experiment Newton allowed a shaft of light (O) from a hole in his window shutter (F) to fall on a glass prism (ABC); the spectrum of light (pqrs) is focussed by a convex lens (MN) and passed through a second prism (DEG). This second prism reconstituted the spectrum as a beam of white light (Y) which then was diffracted through a third prism (HIK) and split again into components (PQRST) projected onto a white screen (LV).

This experiment is often taken to have been definitive proof of Newton's theory of light. However, the experiment was difficult to reproduce and Newton's account was itself a composite of many trials.