  We have measured the contact angle of the interface of phase-separated
$^{3}$He-$^{4}$He mixtures against a sapphire window. We have found that this
angle is finite and does not tend to zero when the temperature approaches
$T_t$, the temperature of the tri-critical point. On the contrary, it increases
with temperature. This behavior is a remarkable exception to what is generally
observed near critical points, i.e. "critical point wetting''. We propose that
it is a consequence of the "critical Casimir effect'' which leads to an
effective attraction of the $^{3}$He-$^{4}$He interface by the sapphire near
$T_{t}$.
