that theory which regards the various phenomena of light as due to undulations in an ethereal medium, propagated from the radiant with immense, but measurable, velocities, and producing different impressions on the retina according to their amplitude and frequency, the sensation of brightness depending on the former, that of color on the latter. The undulations are supposed to take place, not in the direction of propagation, as in the air waves constituting sound, but transversely, and the various phenomena of refraction, polarization, interference, etc., are attributable to the different affections of these undulations in different circumstances of propagation. It is computed that the frequency of the undulations corresponding to the several colors of the spectrum ranges from 458 millions of millions per second for the extreme red ray, to 727 millions of millions for the extreme violet, and their lengths for the same colors, from the thirty-eight thousandth to the sixty thousandth part of an inch. The theory of ethereal undulations is applicable not only to the phenomena of light, but also to those of heat.