  Optical conductivity [$\sigma(\omega)$] of CeRhIn$_5$ and YbNi$_3$Ga$_9$ have
been measured at external pressures to 10 GPa and at low temperatures to 6 K.
Regarding CeRhIn$_5$, at ambient pressure the main feature in $\sigma(\omega)$
is a Drude peak due to free carriers. With increasing pressure, however, a
characteristic mid-infrared (mIR) peak rapidly develops in $\sigma(\omega)$,
and its peak energy and width increase with pressure. These features are
consistent with an increased conduction ($c$)-$f$ electron hybridization at
high pressure, and show that the pressure has tuned the electronic state of
CeRhIn$_5$ from very weakly to strongly hybridized ones. As for YbNi$_3$Ga$_9$,
in contrast, a marked mIR peak is observed already at ambient pressure,
indicating a strong $c$-$f$ hybridization. At high pressures, however, the mIR
peak shifts to lower energy and becomes diminished, and seems merged with the
Drude component at 10 GPa. Namely, CeRhIn$_5$ and YbNi$_3$Ga$_9$ exhibit some
opposite tendencies in the pressure evolutions of $\sigma(\omega)$ and
electronic structures. These results are discussed in terms of the pressure
evolutions of $c$-$f$ hybridized electronic states in Ce and Yb compounds, in
particular in terms of the electron-hole symmetry often considered between Ce
and Yb compounds.
