Descartes believed that he was the first to obtain the law of refraction of light and paid great attention to it as well as to the well-known classical law of reflection. Descartes hypothesized that light is pressure, transmitted instantaneously through a transparent medium. Gassendi, on the contrary, held that light is a stream of tiny particles traveling with immense speed. Newton questioned Descartes' theory in many ways; in folio 103 he wrote—