It helps explain one of the generally accepted physical characteristics of light, that being a speed of 186,000 miles per second. Newton's particle theory, or "corpuscular theory" as it was known then, defined light as tiny particles, or "corpuscles", that traveled in a straight line with a defined velocity. Velocity is speed plus direction. Newton went on to define these particles as "having no need for a medium in which to operate". Newton's particle theory was generally accepted for about one hundred years, but was amended with parts of Huygen's wave theory of light because it simply couldn't satisfy all the physical descriptions of light characteristics.