the other did not obey it, a felicitous and bold hypothesis about the form of the surface of the optical wave in media of crystals" (1906, 35). Hence, on Duhem's view, the credit for Huygens's account of double refraction goes only to the representative part, whereas the wave hypothesis did not contribute at all to this predictive success. Things, however, are more complicated. Duhem himself showed this when he described how Huygens used the Iceland Spar to study double refraction. Huygens observed, as Bartholin did before him, that two images of one and the same line could be produced by placing a Spar on a piece of paper. Moreover, one of these images would rotate if the Spar itself was being rotated. One of these images, that Huygens called "ordinary", obeys Descartes's law and stays fixed when the Spar is rotated. The rotating image, called "extraordinary", does not satisfy the law of refraction. Upon having explained the phenomena of reflection Marie Gueguen and Stathis Psillos Anti-Scepticism and Epistemic Humility in Pierre Duhem's Philosophy of Science 62 and refraction based on a wave hypothesis and the propagation of light through spherical waves, Huygens postulated that the two images were the results of two different kinds of wave propagation, corresponding respectively to the light propagation in the aether contained in the Spar and to the light propagation in the particles constituting the Spar itself. While the former waves are spherical, the latter are ellipsoidal, thus explaining the phenomena of double refraction. Based on these explanatory hypotheses which yielded the ellipsoidal model, Huygens thus constructed a geometrical model that succeeded in representing the phenomena of double refraction and thus extended Descartes's theory of refraction. The point here that Huygens did use the wave hypothesis to construct his theory of double refraction. It was the wave hypothesis that made possible the ellipsoidal model, since as he (1690b, 73) put it: "upon having explained the refraction of transpar