was probably lost. Unfinished business Aether drag and aether density 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 moved at velocity relative to the external aether (taken as stationary), then the velocity of light inside the object gained the additional component . 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 , of which the excess, namely , was dragged along at velocity , whence the average velocity of the internal aether was . The factor in parentheses, which Fresnel originally expressed in terms of wavelengths, became known as the Fresnel drag coefficient. In his analysis of double refraction, Fresnel supposed that the different refractive indices in different directions within the same medium were due to a directional variation in elasticity, not density (because the concept of mass per unit volume is not directional). But in his treatment of partial reflection, he supposed that the different refractive indices of different media were due to different aether densities, not different elasticities. The latter decision, although puzzling in the context of double refraction, was consistent with the earlier treatment of aether drag. In 1846, George Gabriel Stokes pointed out that there was no need to divide the aether inside a moving object into two portions; all of it could be considered as moving at a common velocity. Then, if the aether was conserved while its density changed in proportion to , the resulting velocity of the aether inside the object was equal to Fresnel's additional velocity component. Dispersion The analogy between light waves and transverse waves in elastic solids does not predict dispersion — that is, the frequency-dependence of the speed of propagation, which enables prisms to produce spectra and causes lenses to suffer from chromatic aberration. Fresnel, in De la Lumière and in the second supplement to his first memoir on double refraction, suggested that dispersion could be accounted for if the particles of the medium exerted forces on each other over distances that were significant fractions of a wavelength. Later, more than once, Fresnel referred to the demonstration of this