  We determine the microlensing event rate distribution, $\Gamma(t_{E})$, that
is properly normalized by the first year MACHO group observations (Alcock et
al. 1997). By comparing the determined $\Gamma(t_{E})$ with various MF models
of lens populations to the observed distribution, we find that stars and WDs
explain just $\sim 49%$ of the total event rate and $\sim 37%$ of the
observationally determined optical depth even including very faint stars just
above hydrogen-burning limit. Additionally, the expected time scale
distribution of events caused by these known populations of lenses deviates
significantly from the observed distribution especially in short time scale
region. However, if the rest of the dynamically measured mass of the bulge
($\sim 2.1\times 10^{10} M_\odot$) is composed of brown dwarfs, both expected
and observed event rate distribution matches very well.
