  In conventional superconductors, the most direct evidence of the mechanism
responsible for superconductivity comes from tunneling experiments in which a
clear image of the electron-phonon interaction is revealed. The observed
structure in the current voltage characteristics at the phonon energies can be
used to measure, through inversion of the Eliashberg equations, the electron
phonon spectral density \alpha^2F(\omega). The coherence length in conventional
materials is long and the tunneling process probes several atomic layers into
the bulk of the superconductor. On the contrary, in the high-T_c oxides,
particularly for c-axis tunneling, the coherence length can be quite short and
in an optical experiment or in neutron scattering experiments the bulk of the
sample is probed. Therefore, these spectroscopies become the methods of choice
for the investigation of mechanisms of high-T_c superconductivity. Accurate
reflectance measurements in the infrared range and precise polarized neutron
scattering data are available for a variety of oxides. In this paper we show
that conducting carriers studied by means of infrared spectroscopy reveal
strong coupling to a resonance structure in the spectrum of spin fluctuations
examined with neutron scattering. The coupling strength inferred from
experiment is sufficient to account for high values of T_c which signals the
prominent role of spin excitations in the superconductivity of oxides.
