Descartes on the Refraction and the Velocity of Light

A critical analysis is made of Descartes's ideas concerning the nature of light and its refraction in order to ascertain the logic of his conclusion that light travels with a greater velocity in water than in air. The conclusions are (1) that Descartes was not an adherent of an emission theory but conceived of light as a pressure in a transparent medium, (2) that he believed this pressure was transmitted from the light source at an incredibly rapid but nevertheless finite velocity, and (3) that he was led to his erroneous conclusion concerning the velocity of light in rare and dense media by confusing the pressure and the velocity of the light propagation in the explanation of his experimental results.