  We have performed ellipsometric measurements of the far-infrared c-axis
dielectric response of underdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single
crystals. Here we report a detailed analysis of the temperature-dependent
renormalization of the oxygen bending phonon mode at 320 cm$^{-1}$ and the
formation of the additional absorption peak around 400-500 cm$^{-1}$. For a
strongly underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$ crystal with T$_{c}$=52 K we
find that, in agreement with previous reports based on conventional reflection
measurements, the gradual onset of both features occurs well above T$_{c}$ at
T*$\sim $150 K. Contrary to some of these reports, however, our data establish
that the phonon anomaly and the formation of the additional peak exhibit very
pronounced and steep changes right at T$_{c}$. For a less underdoped
YBa$_{2}$Cu$_{3}$O$_{6.75}$ crystal with T$_{c}$=80 K, the onset temperature of
the phonon anomaly almost coincides with T$_{c}$. Also in contrast to some
previous reports, we find for both crystals that a sizeable fraction of the
spectral weight of the additional absorption peak cannot be accounted for by
the spectral-weight loss of the phonon modes but instead arises from a
redistribution of the electronic continuum. Our ellipsometric data are
consistent with a model where the bilayer cuprate compounds are treated as a
superlattice of intra- and inter-bilayer Josephson-junctions.
