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