“By 1625 he had begun to turn his attention to other mathematical problems and to optics, again clearly in ways that excited the admiration of Mersenne and the ‘erudites’. And rightly so, for at about this time Decartes made an important scientific breakthrough: he discovered the law of refraction. This is the law that gives a geometrical description of the behaviour of rays of light as they pass through the interface between one optical medium and another. In fact, the law had twice been discovered earlier: In 1601 by the English astronomer and mathematician Thomas Harriot, and in 1621 by Willibrord Snell, professor of mathematics at the University of Leiden. Harriot and Snell did not publish their respective discoveries; Harriot’s work has only recently come to be appreciated from a study of his manuscripts, while Snell had better fortune in having his priority over Descartes recognised by Huygens in the 1690s. The law of refraction is now known as Snell’s Law as a result. But Descartes, apart from discovering it independently first gave it …