Who really discovered Snell's law?

Open any physics textbook and you'll soon come across what English-speaking physicists refer to as "Snell's law". The principle of refraction – familiar to anyone who has dabbled in optics – is named after the Dutch scientist Willebrord Snell (1591–1626), who first stated the law in a manuscript in 1621. In French, however, the same law is often called "la loi de Descartes" because it was Rene Descartes (1596–1650) who first put the law into widespread circulation in his Discourse on Method, published in 1637.