  We investigated the electronic structures of the 5$d$ Ruddlesden-Popper
series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$) using optical
spectroscopy and first-principles calculations. As 5$d$ orbitals are spatially
more extended than 3$d$ or 4$d$ orbitals, it has been widely accepted that
correlation effects are minimal in 5$d$ compounds. However, we observed a
bandwidth-controlled transition from a Mott insulator to a metal as we
increased $n$. In addition, the artificially synthesized perovskite SrIrO$_{3}$
showed a very large mass enhancement of about 6, indicating that it was in a
correlated metallic state.
