  With optical spectroscopy we provide evidence that the insulator-metal
transition in Sr$_2$Ir$_{1-x}$Rh$_{x}$O$_{4}$ occurs close to a crossover from
the Mott- to the Slater-type. The Mott-gap at $x = 0$ persists to high
temperature and evolves without an anomaly across the N\'{e}el temperature,
$T_N$. Upon Rh-doping, it collapses rather rapidly and vanishes around $x =
0.055$. Notably, just as the Mott-gap vanishes yet another gap appears that is
of the Slater-type and develops right below $T_N$. This Slater-gap is only
partial and is accompanied by a reduced scattering rate of the remaining free
carriers, similar as in the parent compounds of the iron arsenide
superconductors.
