  We calculate the infrared conductivity of a stack of coupled, two-dimensional
superconducting planes within the Fermi liquid theory of superconductivity. We
include the effects of random scattering processes and show that the presence
of even a small concentration of resonant impurities, in a $d$-wave
superconductor, has an important effect on both the in-plane and $c$-axis
transport properties, which could serve as signatures for $d$-wave pairing.
