  Nuclear-size corrections of order $(Z \alpha)^5$ and $(Z \alpha)^6$ to the
S-state levels of hydrogenic atoms are considered. These nuclear-elastic
contributions are somewhat smaller than the polarizability (nuclear-inelastic)
corrections for deuterium, but are of comparable or larger size for the
hydrogen case. For deuterium the (attractive) nonrelativistic $(Z \alpha)^5$
correction to the 2S-1S transition is 0.49 kHz, while the (repulsive)
relativistic $(Z \alpha)^6$ contribution is -3.40 kHz. For the proton the
corresponding corrections are 0.03 kHz and -0.61 kHz, respectively. The $(Z
\alpha)^5$ contribution largely cancels the Coulomb-retardation part of the
nuclear-polarization correction.
