In 1648, J. Marcus Marci of Prague anticipated two chief features of Isaac Newton's celebrated 1672 theory of light and color, namely that colors are inherent to light and that the role of the prism is to separate the rays of color by means of refraction. Furthermore, Marci argued that colors produced by a first refraction are immutable when subjected to refraction by a second prism. This paper argues that the key to Marci's achievement derived from his chymical view of (...) light, which he tested by means of prism trials and geometrical constructions. I also suggest that Marci's unusual coupling of chymical philosophy with mathematics was a move unusual in the history of alchemy; and that his chymical understanding of light belongs to an underexplored tradition in the history of optics, which is distinct from mechanistic and Aristotelian theories of light. (shrink)