1. Bethe H. A.,Phys. Rev.,72 (1947); 339;Levinger J. S. andBethe H. A.,Phys. Rev.,78 (1950) 115.
2. Del Guidice E., Preparata G. andVitiello G.,Phys. Rev. Lett.,61 (1988) 1085. See alsoDel Giudice E., Giuffrida, M., Mele R. andPreparata G.,Europhys. Lett.,14 (1991) 463.
3. We recall that successful Lamb-shift calculations have been performed by quasi-classical methods. SeeBarut A. A. andVan Huele J. F.,Phys. Rev. A,32 (1985) 3187 and references quoted there.
4. A very clear presentation of QED in Coulomb gauge can be found,e.g., in the Appendix ofCohen-Tannoudji C., Dupont-Roc J. andGrynberg G.,Atom-Photon Interactions: Basic Processes and Applications (Wiley, New York, N. Y.) 1992.
5. In actual paramagnets the orbital angular momentumL i of eq. (3.13) is (partially) quenched owing to the interaction of the ion with the crystal field (anisotropic Coulomb field). In consequence of this fact, the Landé factorg is not the same as for a free ion. This effect indicates that a non-intracting ion approximation is really a rough one in the description of an actual paramagnetic substance.