Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/449

 denotes the refractive index of the dielectric; and therefore the refractive index is determined in terms of the frequency by the equation

This formula is equivalent to that which Maxwell and Sellmeier had derived from the elastic-solid theory. Though superficially different, the derivations are alike in their essential feature, which is the assumption that the molecules of the dielectric contain systems which possess free periods of vibration, and which respond to the oscillations of the incident light. The formula may be derived on electromagnetic principles without any explicit reference to electrons; all that is necessary is to assume that the dielectric polarization has a free period of vibration.

When the luminous vibrations are very slow, so that is small,  reduces to the dielectric constant  ; so that the theory of Lorentz leads to the expression