Reflection on refraction

An attempt, based mainly on his correspondence, is made to reconstruct Descartes' discovery of the law of refraction. Descartes conjectured the law from purely metaphysical, geometrical, and a priori assumptions. He performed no measurements which could serve him as a basis for induction or generalization, and in fact it would be impossible. He attempted no verifications, instead he devised only one crucial experiment: he deduced from his law the form of the anaclastic curve and asked Ferrier to cut one hyperbolic lens. Then he proclaimed: if this lens does not concentrate light beams into one point, then not only his law, bul all his philosophy will be worthless. Descartes' story fits perfectly into Popper's Logic of Scientific Discovery and the crucial experiment can serve as a perfect example of a potential falsifier.