in every Ray apart, so far as appears by Experiment. And that it is accurately true, may be demonstrated upon this Supposition. _That Bodies refract Light by acting upon its Rays in Lines perpendicular to their Surfaces._ But in order to this Demonstration, I must distinguish the Motion of every Ray into two Motions, the one perpendicular to the refracting Surface, the other parallel to it, and concerning the perpendicular Motion lay down the following Proposition. If any Motion or moving thing whatsoever...