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Attempts in antiquity and the Middle Ages to determine the mathematical law of refraction are well known. In view of the movement toward the mathematization of physical laws, which has made great gains since the beginning of the seventeenth century, and of the efforts of Hariot, Kepler, Snell, and Descartes to determine the true mathematical ratio between the angles of incidence and refraction, it is understandable that historians of pre-seventeenth-century science should concentrate on the quantitative aspects of refraction. But to (...) do so is to gain a distorted picture of early optical thought, for as much effort was actually devoted to understanding the cause of refraction as to finding the mathematical law of refraction. (shrink)