Linear dispersive dielectrics as limits of Drude-Lorentz systems.

We investigate the limiting case of the Drude-Lorentz model for the complex electric permeability epsilon(omega)=1+chi;(omega) as the damping tends to zero. We find that epsilon(omega) becomes real except for a number of discrete frequencies. The Kramers-Kronig relations connecting the real and imaginary parts of chi;(omega) remain valid.