Adapting an approach used by Feynman for the Lamb shift of an isolated atom
, we obtain a nonperturbative expression for the Lamb shift of an atom in a
dielectric medium, previously calculated perturbatively [P. W. Milonni, J.
Mod. Opt. 42, 1191 (1995)]. The separation of the Lamb and Casimir compone
nts of the field energy is greatly simplified, and previous nonperturbative
results for the Lamb shift [M. Schaden, L. Spruch, and F. Zhou, Phys. Rev.
A 57, 1108 (1998)], based on the generalized argument theorem, are obtaine
d much more easily and directly. [S1050-2947(99)12205-4].