On the Velocity of Light as Determined by Foucault's Revolving Mirror

Abstract

A FEW years ago Lord Rayleigh raised an interesting question as to the quantity actually determined by our experiments and observations on the velocity of light. There can be no difficulty as long as the medium transmits different wave-lengths with the same velocity, but whenever the medium possesses the property of dispersion the velocity with which any one crest of a wave travels (V) is different from the velocity with which a group of waves is propagated (U); hence the question arises in each particular case, whether it is V or U or something depending on both quantities that we measure.