  The contribution of virtual excitations to the energy levels of electronic
and muonic hydrogen is investigated combining a model-independent approach for
the main part with quark model predictions for the remaining corrections.
Precise values for the polarization shifts are obtained in the long-wavelength
dipole approximation by numerically integrating over measured total
photoabsorption cross sections. These unretarded results are considerably
reduced by including retardation effects in an approximate way since the
average momentum transfer (together with the mean excitation energy) turns out
to be larger than usually assumed. Transverse and seagull contributions are
estimated in a simple harmonic oscillator quark model and found to be
non-negligible. Possible uncertainties and improvements of the final results
are discussed.
