  Continuing a program of examining the behavior of the vacuum expectation
value of the stress tensor in a background which varies only in a single
direction, we here study the electromagnetic stress tensor in a medium with
permittivity depending on a single spatial coordinate, specifically, a planar
dielectric half-space facing a vacuum region. There are divergences occurring
that are regulated by temporal and spatial point-splitting, which have a
universal character for both transverse electric and transverse magnetic modes.
The nature of the divergences depends on the model of dispersion adopted. And
there are singularities occurring at the edge between the dielectric and vacuum
regions, which also have a universal character, depending on the structure of
the discontinuities in the material properties there. Remarks are offered
concerning renormalization of such models, and the significance of the stress
tensor. The ambiguity in separating "bulk" and "scattering" parts of the stress
tensor is discussed.
