  We compute the expectations of the squares of the electric and magnetic
fields in the vacuum region outside a half-space filled with a uniform
non-dispersive dielectric. This gives predictions for the Casimir-Polder force
on an atom in the `retarded' regime near a dielectric. We also find a positive
energy density due to the electromagnetic field. This would lead, in the case
of two parallel dielectric half-spaces, to a positive, separation-independent
contribution to the energy density, besides the negative, separation-dependent
Casimir energy. Rough estimates suggest that for a very wide range of cases,
perhaps including all realizable ones, the total energy density between the
half-spaces is positive.
