  We report the complex optical conductivity of a superconducting thin-film of
Nb$_{0.5}$Ti$_{0.5}$N in an external magnetic field. The field was applied
parallel to the film surface and the conductivity extracted from far-infrared
transmission and reflection measurements. The real part shows the
superconducting gap, which we observe to be suppressed by the applied magnetic
field. We compare our results with the pair-breaking theory of Abrikosov and
Gor'kov and confirm directly the theory's validity for the optical
conductivity.
