This paper argues that Kepler considered his work in optics as part of natural philosophy and that, consequently, he aimed at change within natural philosophy. Back-to-back with John Schuster’s claim that Descartes’ optics should be considered as a natural philosophical appropriation of innovative results in the tradition of practical and mixed mathematics the central claim of my paper is that Kepler’s theory of optical imagery, developed in his Paralipomena ad Vitellionem (1604), was the result of a move similar to Descartes’ (...) by Kepler. My argument consists of three parts. First, Kepler borrowed a geometrical model and experiment of optical imagery from the mélange of mixed and practical mathematics provided in the works of the sixteenth-century mathematicians Ettore Ausonio and Giovanni Battista Della Porta. Second, Kepler criticized the Aristotelian theory of light and he developed his own alternative metaphysics. Third, Kepler used his natural philosophical assumptions about the nature of light to re-interpret the model of image formation taken from Della Porta’s work. Taken together, I portray Kepler’s theory of optical imagery as a natural philosophical appropriation of an innovative model of image formation developed in a sixteenth-century practical and mixed mathematical tradition which was not interested in questioning philosophical assumptions on the nature of light. (shrink)

There is an uncanny unanimity about the founding role of Kepler's Dioptrice in the theory of optical instruments and for classical geometric optics generally. It has been argued, however, that for more than fifty years optical theory in general, and Dioptrice in particular, was irrelevant for the purposes of telescope making. This article explores the nature of Kepler's achievement in his Dioptrice . It aims to understand the Keplerian 'theory' of the telescope in its own terms, and particularly its links (...) to Kepler's theory of vision. It deals first with Kepler's way to circumvent his ignorance of the law of refraction, before turning to Kepler's explanations of why lenses magnify and invert vision. Next, it analyses Kepler's account of the properties of telescopes and his suggestions to improve their designs. The uses of experiments in Dioptrice , as well as the explicit and implicit references to della Porta's work that it contains, are also elucidated. Finally, it clarifies the status of Kepler's Dioptrice vis-à-vis , classical geometrical optics and presents evidence about its influence in treatises about the practice of telescope making during roughly the first two-thirds of the seventeenth century. (shrink)