ABSTRACT
In his Dioptrique of 1637 Descartes raised numerous difficulties and puzzles. For example, he deduced the laws of reflection and refraction from a model: the motion of some very curious tennis balls. Descartes’ contemporaries tended not to see any cogency in this model, nor did they grasp the theory of dynamics upon which it is based.2 Later, questions were raised about how Descartes had obtained the law, if not through his dubious deduction. Had Descartes plagiarised it from Willebrord Snel? If not, where had it come from?3
