  We take advantage of the connection between the free carrier optical
conductivity and the glue function in the normal state, to reconstruct from the
infrared optical conductivity the glue-spectrum of ten different high-Tc
cuprates revealing a robust peak in the 50-60 meV range and a broad continuum
at higher energies for all measured charge carrier concentrations and
temperatures up to 290 K. We observe an intriguing correlation between the
doping trend of the experimental glue spectra and the critical temperature.
