  We demonstrate that a very recently obtained formula for the force on a slab
in a material planar cavity based on the calculation of the vacuum Lorentz
force [C. Raabe and D.-G. Welsch, Phys. Rev. A 71, 013814 (2005)] describes a
(medium) screened Casimir force and, in addition to it, a medium-assisted
force. The latter force also describes the force on the cavity medium. For
dilute media, it implies the atom-mirror interaction of the Casimir-Polder type
at large and of the Coulomb type at small atom-mirror distances of which the
sign is insensitive to the polarizability type (electric or magnetic) of the
atom.
