A method is suggested by which the refractive index and Verdet constant of an atomic system may be derived theoretically. It is applied to atomic hydrogen and to the inert gases and a comparison is made with experimental data. The Verdet constant of neon is not anomalous. The origin of the suggestion appears to be an underestimate of the experimental error. The analysis yields values of the polarizabilities of the inert gases which are respectively He, 1.384; Ne, 2.663; Ar, 11.080; Kr, 16.734; Xe, 27.292 in units of a$^3_0$.