 Kepler contributed to the special field of optics with two seminal works, the Ad Vitellionem paralipomena (=APO) and the Dioptrice (=DI), the latter motivated in large part by the publication in 1610 of Galileo’s Sidereal Messenger (Sidereus Nuncius). In his Conversation with the Sidereal Messenger (Dissertatio cum Nuncio Sidereo … a Galillaeo Galilaeo, KGW 4, pp. 281–311), he supported the factual information given by Galileo, indicating at the same time the necessity of giving an account of the causes of the observed phenomena. The background for his investigation into optics was undoubtedly the different particular questions of astronomical optics (see Straker 1971). In this context he concentrated his efforts on an explanation of the phenomena of eclipses, of the apparent size of the Moon and of atmospheric refraction. Kepler investigated the theory of the camera obscura very early and recorded its general principles (see commentary by M. Hammer in KGW 2, pp. 400–1 and Straker 1981). In addition, he worked intensively on the theory of the telescope and invented the refracting astronomical or ‘Keplerian’ telescope, which involved a considerable improvement over the Galilean telescope (see especially DI, Problem 86, KGW 4, pp. 387–88). Besides these impressive contributions, Kepler expanded his research program to embrace mathematics as well as anatomy, discussing for instance conic sections and explaining the process of vision (see Crombie 1991 and especially Lindberg 1976b).