  A recent analysis of optical conductivity data which has provided strong
evidence for coupling of the charge carriers to the 41 meV spin resonance, seen
in the superconducting state of optimally doped Y123, is extended to other
systems. We find that the corresponding spin resonance is considerably broader
in Tl2201 and Y124 than it is in Bi2212. On the other hand, in overdoped Tl2201
with T_c = 23 K no resonance forms. A more specific interpretation of the
optical data in terms of a spin fluctuation exchange mechanism of
superconductivity gives an underlying effective spectral density which is
strongly temperature dependent and which contains important feedback effects
that further stabilize superconductivity as the temperature is reduced.
