Virtually all survey-accounts of ancient science leave the mistaken impression that ancient ray-theory shared the same basic aims and methods as its modern counterpart and, therefore, that the two are genetically linked. The source of confusion lies in the fact that, for the sake of simplicity and brevity, textbook authors emphasize those aspects of Greek ray-analysis that seem most innovative and forward-thinking. Viewed in this way, Greek ray-theory looks very much like its modern countrpart. In reality, ancient and modern ray-theory are worlds apart in conceptual and methodological foundations, and the same holds true for their fundamental aims. The history of ancient optics has undergone a fundamental reappraisal in regard to both the basic sources and their interpretive context. Using actual sources as a foundation, Mark Smith shows how ancient mathematical optics developed in response not only to certain theoretical imperatives but also to empirical evidence, and opens a path for readers to reach their own understanding.