  Measurements of Casimir effects in $^4$He films in the vicinity of the bulk
superfluid transition temperature $T_\lambda$ have been carried out, where
changes in the film thickness and the superfluid density are both monitored as
a function of temperature. The Kosterlitz-Thouless superfluid onset temperature
in the film is found to occur just as the Casimir dip in the film thickness
from critical fluctuations becomes evident. Additionally, a new film-thickening
effect is observed precisely at $T_\lambda$ when the temperature is swept
extremely slowly. We propose that this is a non-universal Casimir effect
arising from the viscous suppression of second sound modes in the film.
