  We review some previous studies concerning the intra-bilayer Josephson
plasmons and present new ellipsometric data of the c-axis infrared response of
almost optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8}. The c-axis conductivity of
this compound exhibits the same kind of anomalies as that of underdoped
YBa_{2}Cu_{3}O_{7-delta}. We analyze these anomalies in detail and show that
they can be explained within a model involving the intra-bilayer Josephson
effect and variations of the electric field inside the unit cell. The Josephson
coupling energies of different bilayer compounds obtained from the optical data
are compared with the condensation energies and it is shown that there is a
reasonable agreement between the values of the two quantities. We argue that
the Josephson coupling energy, as determined by the frequency of the
intra-bilayer Josephson plasmon, represents a reasonable estimate of the change
of the effective c-axis kinetic energy upon entering the superconducting state.
It is further explained that this is not the case for the estimate based on the
use of the simplest ``tight-binding'' sum rule. We discuss possible
interpretations of the remarkable agreement between the Josephson coupling
energies and the condensation energies. The most plausible interpretation is
that the interlayer tunneling of the Cooper pairs provides the dominant
contribution to the condensation energy of the bilayer compounds; in other
words that the condensation energy of these compounds can be accounted for by
the interlayer tunneling theory. We suggest an extension of this theory, which
may also explain the high values of T_{c} in the single layer compounds
Tl_{2}Ba_{2}CuO_{6} and HgBa_{2}CuO_{4}, and we make several experimentally
verifiable predictions.
