The "Dioptrice" is available here in a complete English translation for the first time, along with an introduction and numerous notes to help guide the reader through Kepler's text. Kepler's "Dioptrice" is a seminal work in the fields of optics and astronomy. Written in response to the ground-breaking telescopic discoveries announced by Galileo Galilei in March 1610, the "Dioptrice" contained the first theory of the telescope—a theory that Kepler was uniquely qualified to develop—and provided an essential foundation for modern geometrical optics. In the preface, Kepler highlights several examples of the close links which, then as now, exist between optics and astronomy. His excitement about Galileo's new observations is palpable, especially the discoveries of the four moons orbiting Jupiter and the phases of Venus. Both discoveries lent crucial support to the Copernican system, of which Kepler had been an ardent supporter since his days as a student in Tübingen. The "Dioptrice" is available here in a complete English translation for the first time, along with an introduction and numerous notes to help guide the reader through Kepler's text. Table of contents Translator’s Introduction 7 The invention of the telescope 7 Kepler’s path to the Dioptrice 9 The telescope and Galileo’s discoveries 16 Contents of the Dioptrice 20 Publication and impact 25 Notes on the translation 29 Final remarks 36 Acknowledgements 38 Kepler’s Dioptrice 41 Dedication 43 Preface 47 Dioptrics 82 Felix Platter’s illustrations of the eye 169 Bibliography 170