  We summarize existing optical data of URu$_2$Si$_2$ to clarify the nature of
the hidden order transition in this heavy fermion metal. Hybridization develops
between 50 K and 17.5 K, and a coherent Drude peak emerges which mirrors the
changes in the dc resistivity. The Drude weight indicates that there is little
change in the effective mass of these carriers in this temperature range. In
addition, there is a flat background conductivity that develops a partial
hybridization gap at 10 meV as the temperature is lowered, shifting spectral
weight to higher frequencies above 300 meV. Below 30 K the carriers become
increasingly coherent and Fermi-liquid-like as the hidden order transition is
approached. The hidden order state in URu$_2$Si$_2$ is characterized by
multiple anisotropic gaps. The gap parameter $\Delta_a=3.2$ meV in the {\it
ab}-plane. In the {\it c}-direction, there are two distinct gaps with
magnitudes of $\Delta_{c1}=2.7$ meV and $\Delta_{c2}=1.8$ meV. These
observations are in good agreement with other spectroscopic measurements.
Overall, the spectrum can be fit by a Dynes-type density of states model to
extract values of the hidden order gap. The transfer of spectral weight
strongly resembles what one sees in density wave transitions.
