Supposing the phenomena of light to be caused by the indefinitely small vibrations of a highly elastic medium pervading space, it is a simple problem to determine the motion of such a medium in vacuo, or in space, where matter does not exist, as in these cases the problem is reduced to the determination of the motion of a homogeneous elastic medium.

On proceeding, however, to investigate the motion of the ethereal medium in crystals, for the purpose of accounting for the phenomena of crystalline refraction, the question arises, whether there is an action between the material molecules and the etherial medium. In other words, are the laws of the refraction of the ether within crystals, independent of the existence of material molecules, so that the ether may be treated as a single elastic medium, or are the phenomena of crystalline refraction produced, wholly or partially, by a direct action between the material molecules and the ether?