  We present observations of the insulator-superconductor transition in
ultrathin films of Bi on amorphous quartz, quartz coated with Ge, and for the
first time, solid xenon condensed on quartz. The relative permeability
$\epsilon_r$ ranges from 1.5 for Xe to 15 for Ge. Though we find screening
effects as expected, the I-S transition is robust, and unmodified by the
substrate. The resistance separatrix is found to be close to h/4e^2 and the
crossover thickness close to $\rm 25 \AA$ for all substrates. I-V studies and
Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The
transition can be understood in terms of a percolation model.
