  We investigate nonresonant contributions to resonant Rayleigh scattering
cross sections of atoms. The problematic nonresonant contributions set a limit
to the accuracy to which atomic spectra determine energy levels. We discuss the
off-resonance effects in one-photon transitions. We also show that
off-resonance contributions for the 1S-2S two-photon transition in atomic
hydrogen are negligible at current and projected levels of experimental
accuracy. The possibility of a differential measurement for the detection of
off-resonance effects in one-photon transitions in atomic hydrogen is
discussed.
