Quasielastic electron scattering and vacuum polarization.

The longitudinal response function for quasielastic electron scattering from /sup 12/C and /sup 40/Ca at a momentum transfer q = 550 MeV is calculated in a relativistic random-phase approximation (RPA). The particle-hole response is calculated exactly in the finite system while vacuum-polarization effects are included in a local-density approximation. The RPA result with vacuum polarization is about 25% below the Hartree response and comparable to Saclay data.