Professor Newcomb's Determination of the Velocity of Light ¹

Abstract

THE method selected for the important experiments described in the present memoir,2 is that known as Foucault's. The idea fundamental to it is that of the determination of the interval occupied by light in flashing-from a revolving to a fixed mirror and back, by the amount of deviation produced in its return path through the change meantime effected in the position of the revolving mirror. The angle of deviation of the ray is double the angle of displacement of the reflector; to this angle corresponds (since the mirror rotates at a known rate) a definite fraction of a second, which is the time of luminous transmission across twice the measured distance between the mirrors.