Descartes discovered an early form of the law of conservation of momentum (a measure of the motion of an object), and envisioned it as pertaining to motion in a straight line, as opposed to perfect circular motion, as Galileo had envisioned it. He outlined his views on the universe in his Principles of Philosophy, where he describes his three laws of motion. (Newton's own laws of motion would later be modeled on Descartes' exposition.) Descartes also made contributions to the field of optics. He showed by using geometric construction and the law of refraction (also known as Descartes' law, or more commonly Snell's law outside France) that the angular radius of a rainbow is 42 degrees (i.e., the angle subtended at the eye by the edge of the rainbow and the ray passing from the sun through the rainbow's centre is 42°). He also independently discovered the law of reflection, and his essay on optics was the first published mention of this law. Influence on Newton's mathematics Current popular opinion holds that Descartes had the most influence of anyone on the young Isaac Newton, and this is arguably one of his most important contributions. Decartes' influence extended not directly from his original French edition of La Géométrie, however, but rather from Frans van Schooten's expanded second Latin edition of the work. Newton continued Descartes' work on cubic equations, which freed the subject from fetters of the Greek perspectives. The most important concept was his very modern treatment of single variables. Newton rejected Descartes' vortex theory of planetary motion in favor of his law of universal gravitation, and most of the second book of Newton's Principia is devoted to his counterargument. Contemporary reception In commercial terms, The Discourse appeared during Descartes' lifetime in a single edition of 500 copies, 200 of which were set aside for the author. Sharing a similar fate was the only French edition of The Meditations, which had not managed to sell out by the time of Descartes' death. A concomitant Latin edition of the latter was, however, eagerly sought out by Europe's scholarly community and proved a commercial success for Descartes. Although Descartes was well known in academic circles towards the end of his life, the teaching of his works in schools was controversial. Henri de Roy (Henricus Regius, 1598–1679), Professor of Medicine at the University of Utrecht, was condemned by the Rector of the university,