Snell or Snellius, Willebrord (vĭlˈəbrôrt snĕl, snĕlēəs) [key], 1591–1626, Dutch mathematician. He is generally credited with the discovery (1621) of the law of the refraction of light. In 1613 he became professor of mathematics at the Univ. of Leiden. His two chief works are Eratosthenes Batavus (1617) and Cyclometricus (1621).