In 1810, Arago found experimentally that the degree of refraction of starlight does not depend on the direction of the earth's motion relative to the line of sight. In 1818, Fresnel showed that this result could be explained by the wave theory, on the hypothesis that if an object with refractive index n moved at velocity v relative to the external aether (taken as stationary), then the velocity of light inside the object gained the additional component v ( 1 − − 1 / n 2 ) . He supported that hypothesis by supposing that if the density of the external aether was taken as unity, the density of the internal aether was n 2 , of which the excess, namely n 2 − − 1 , was dragged along at velocity v , whence the average velocity of the internal aether was v ( 1 − − 1 / n 2 ) . The factor in parentheses, which Fresnel originally expressed in terms of wavelengths, became known as the Fresnel drag coefficient. (See Aether drag hypothesis.)