This essay on meteorology contains Descartes’ explanation of the optics of the rainbow and his law of refraction. Descartes’ ambitious aim was to produce a new body of writings that would completely displace the Aristotelian corpus. Against Aristotle, but like Seneca, he argued that even in meteorology, certain knowledge lies within reach if one employs a sound methodology. In the Discourse on Method, Descartes championed the mechanical philosophy against both Aristotle and the chemical philosophy of Paracelsus. It includes three appendices to show his approach to science and mathematics: La Dioptrique, Les Météores, and La Géometrie.