2-LOOP LADDER DIAGRAM FOR THE VACUUM POLARIZATION CONTRIBUTION IN HYDROGEN-LIKE IONS

We discuss a calculation scheme for the two-loop ladder graph vacuum p olarization contribution to the Lamb shift in hydrogen-like heavy ions . It is obtained by including the vacuum polarization potential self-c onsistently into the Dirac equation. Numerical values are presented fo r all systems with nuclear charge numbers Z greater than or equal to 1 0. Possible limitations of our method are mentioned. In addition, we a lso present numerical values for the Wichmann-Kroll contribution to th e Lamb shift for all hydrogen-like systems utilizing an extended nucle ar charge distribution in the Z-range 30-100.