  By combining SIM observations with ground-based photometry, one can
completely solve microlensing events seen toward the Galactic bulge. One could
measure the mass, distance, and transverse velocity of ~100 lenses to ~5%
precision in only ~500 hours of SIM time. Among the numerous applications are
1) measurement of the mass functions (MFs) of the bulge and disk 2) measurement
of the relative normalizations of the bulge and disk MFs (and so their relative
contribution to the Galactic potential), 3) measurement of the number of bulge
white dwarfs and neutron stars (and so the initial MF well above the present
turnoff). SIM astrometric measurements are simultaneously photometric
measurements. SIM astrometry determines the angular size of the Einstein ring
on the sky, and comparison of SIM and ground-based photometry determines the
size of the Einstein ring projected onto the observer plane. Only by combining
both of these measurements is it possible to completely solve the microlensing
events.
