  Emphasizing first the utility of the generalized Fresnel coefficients in the
theory of the Casimir effect in planar cavities, we complement our previous
discussion of the ordinary Casimir force on and the Casimir stress in a metal
(plasma) slab in a planar cavity. We demonstrate strong dependence of the
Casimir stress in a thin slab on properties of the bounding medium in the
symmetric Lifshitz configuration. Contrary to this, the stress in a thick slab
gradually becomes insensitive on external boundary conditions. We also consider
the position dependence of the Casimir force on and stress in a thin metal slab
in a planar cavity. Whereas the force per unit area on the slab strongly
increases when it approaches a mirror the stress in the slab decreases and
eventually changes the sign. Generally, the stress decreases with the cavity
width and decreasing reflectivity of the mirrors.
