  Infrared reflectivity measurements, using p-polarized light at a grazing
angle of incidence, show an increased sensitivity to the optical conductivity
of highly reflecting superconducting materials. We demonstrate that when this
measurement technique is applied to the conventional s-wave superconductor NbN,
the results are in perfect agreement with BCS theory. For the in-plane response
of a La$_{1.85}$Sr$_{0.15}$CuO$_4$ single crystal, in the superconducting
state, we find a reduction of the optical conductivity in the frequency range
below 20 meV. The observed frequency dependence excludes an isotropic s-wave
gap, but agrees well with model calculations assuming a d-wave order parameter.
