§ 1. Let us suppose that a light wave is being propagated and absorbed in a homogeneous medium. Take OY as the direction of propagation, and consider a slice of the medium bounded by two planes at right angles to the direction of propagation and distant dy apart; dy is small in comparison with the wave-length. Let x=f(t) denote the mean displacement of the electrons, and let X = A cos gt denote the electric intensity of the light wave in the slice, measured in electrostatic units. Then the average rate at which work is being done on an electron is