  We present measurements of the optical spectra on single crystals of
spinel-type compound \cis. This material undergoes a sharp metal-insulator
transition at 230 K. Upon entering the insulating state, the optical
conductivity shows an abrupt spectral weight transfer and an optical excitation
gap opens. In the metallic phase, Drude components in low frequencies and an
interband transition peak at $\sim 2 eV$ are observed. In the insulating phase,
a new peak emerges around $0.5 eV$. This peak is attributed to the transition
of electrons from the occupied Ir$^{3+}$ $t_{2g}$ state to upper Ir$^{4+}$
$t_{2g}$ subband resulting from the dimerization of Ir$^{4+}$ ions in
association with the simultaneous formations of Ir$^{3+}$ and Ir$^{4+}$
octamers as recently revealed by the x-ray diffraction experiment. Our
experiments indicate that the band structure is reconstructed in the insulating
phase due to the sudden structural transition.
