  Casimir forces are a manifestation of the change in the zero-point energy of
the vacuum caused by the insertion of boundaries. We show how the Casimir force
can be efficiently computed by consideration of the vacuum fluctuations that
are suppressed by the boundaries, and rederive the scalar Casimir effects for a
series of the Dirichlet geometries. For the planar case a finite universal
force is automatically found. Consistent with other calculations of the effect,
for curved geometries divergent (non-universal) expressions are encountered.
They are interpreted geometrically following Candelas and Deutsch (1979) as
largely due to the divergent self-energy of the boundary contributing to the
force. This viewpoint is supported by explicit calculations for a
wedge-circular arc geometry in two dimensions where non-universal and universal
contributions into the effect can be unambiguously separated. We also give a
heuristic derivation of the purely geometrical expression (Sen, 1981) for the
non-universal piece of the Casimir energy due to an arbitrary smooth
two-dimensional Dirichlet boundary of a compact region.
