This chapter reconstructs the genealogy of Descartes' discovery of the law of refraction; initial development of a theory of lenses; and first attempts, in the years 1626–1628, to explain the law through a mechanistic theory of light. These events of the mid to late 1620s constitute the greatest of Descartes' achievements in mixed- and physico-mathematics. They were also of the utmost importance for his emergence, from the late 1620s, as a systematic corpuscular-mechanical natural philosopher. He would use the discovery of the law of refraction as a putative example of his supposedly all conquering method. More importantly, the optical work led him to the mature formulation of the central concepts of his dynamics—the causal register of his emerging system of corpuscular-mechanism, when he later came to write Le Monde. Thus, his optical triumph of the 1620s was both the climax of his early physico-mathematical agenda, as well as the exemplar for important parts of his mature, systematic natural philosophical work to come.