  We have studied the optical properties of four
(LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ superlattices (SL) ($n$=2, 3, 4, 5) on
SrTiO$_3$ substrates. We have measured the reflectivity at temperatures from 20
K to 400 K, and extracted the optical conductivity through a fitting procedure
based on a Kramers-Kronig consistent Lorentz-Drude model. With increasing
LaNiO$_3$ thickness, the SLs undergo an insulator-to-metal transition (IMT)
that is accompanied by the transfer of spectral weight from high to low
frequency. The presence of a broad mid-infrared band, however, shows that the
optical conductivity of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ SLs is not a linear
combination of the LaMnO$_3$ and LaNiO$_3$ conductivities. Our observations
suggest that interfacial charge transfer leads to an IMT due to a change in
valence at the Mn and Ni sites.
