  We outline a procedure for obtaining the electron-phonon spectral density by
inversion of optical conductivity data, a process very similar in spirit to the
McMillan-Rowell inversion of tunelling data. We assume both electron-impurity
(elastic) and electron-phonon (inelastic) scattering processes. This procedure
has the advantage that it can be utilized in the normal state. Furthermore, a
very good qualitative result can be obtained explicitly, without iteration. We
illustrate this technique on recently acquired far-infrared data in
K$_3$C$_{60}$. We show that the electron-phonon interaction is most likely
responsible for superconductivity in these materials.
