Isaac Newton

“ The least Light or part of Light, which may be stopped alone without the rest of the Light, or propagated alone, or do or suffer any thing alone; which the rest of the Light doth not or suffers not, I call a Ray of Light. ”

“ III Refexibility of Rays, is their Disposition to be reflected or turned back into the same Medium from any other Medium upon the whole Surface they fall. And Rays are more or less reflexible, which are turned back more or less easily. As if Light pass out of a Glass into Air, and by being inclined more and more to the common Surface of the Glass and Air, begins at length to be totally reflected by that Surface: those sorts of Rays which at like Incidences are reflected most copiously, or by inclining the Rays begin soonest to be totally reflected, are most reflexible. ”

“ V The Angle of Reflexion or Refraction, is the Angle which the line described by the reflected or refracted Ray containeth with the Perpendicular to the reflecting or refracting Surface at the Point of Incidence. ”