The variation-perturbation method for time-dependent problems is applied to many-electron systems. Specific formulas are developed for the steady-state dipolar interaction between an oscillating electric field and an atom or molecule whose wavefunction is approximated by a Hartree-Fock determinant. The method is illustrated by a number of calculations for the helium atom. Theoretical expressions for the dynamic polarizability, refractive index, and Verdet constant (Faraday effect) are obtained. Comparison with the available measurements shows good agreement between the experimental and the theoretical values.