  Finite size effect in critical systems can be interpreted as universal
Casimir forces. Here we compare the Casimir force for free, periodic and
antiperiodic boundary conditions in the exactly calculable case of the Ising
model in one and two dimensions. We employ a new procedure which allows us to
calculate the Casimir force with periodic and antiperiodic boundary conditions
analytically in a transparent manner. Quite interestingly, we find an
attractive Casimir force for the case of periodic boundary condition but a
repulsive Casimir force for the anti-periodic one.
