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We present an overview of the contributions to the isotope shift of the 1S-2S transition between hydrogen and deuterium. We have calculated an additional contribution to the energy, due to the exchange of a virtual transverse photon between the electron and the deuteron. We also correct the previously evaluated finite-size and recoil contributions [F. Schmidt-Kaler et ol. , Phys. Rev. Lett. 70, 2261]. Our result is 670 994 445(33) kHz, which is in disagreement with the experimental value of 670994337(22) kHz. This discrepancy is probably caused by an incorrect value for the deuteron radius.PACS number(s): 03.65.Bz, 12.20.Ds

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The frequencies of four ' Te& transitions at 486 nm, which are of interest as references in precision measurements of the 1S-2S transitions in hydrogen, deuterium, and positronium, are determined with an accuracy of 1 part in 10 by interferometric comparison with '"I& standard transitions.The spectroscopy of simple one-electron systems plays a major role in the testing of basic physical theories such as quantum electrodynamics, and in the measurement of fundamental constants. In particular, transitions of narrow linewidth are of interest. This has led to the study of the 1S-2Stwo-photon transitions in hydrogen, ' deuterium, positronium, and muonium.The absolute frequencies of each of these transitions can be determined by comparison with Doppler-free transitions in molecular tellurium vapor (' Tez) in the 486-nm region. For exam-