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(Abridged version): The optical depth is widely used in Galactic microlensingstudies as a means to determine the density of MACHOs, since in theory itdepends only upon their spatial distribution and is therefore lessmodel-dependent than other microlensing observables. However, since themeasured optical depth is restricted to that of events with measurabletimescales, inferences of total MACHO density are dependent on the assumedtimescale distribution. Using the 2-year LMC results of the MACHOcollaboration, we illustrate this point by showing how, for an assumedisothermal halo, the inferred MACHO halo fraction as determined from opticaldepth estimates depends upon MACHO mass.  The analysis highlights the following conclusions: (1) The MACHO densityinferred from optical depth measurements depends on the assumed MACHO massfunction for a given Galactic distribution function; (2) without extrainformation on the MACHO mass function, such as can be obtained from therate-timescale distribution, optical depth measurements can provide a lowerlimit but not an upper limit on the MACHO density for a given Galacticdistribution function; (3) a comparison between the inferred total opticaldepth of different Galactic models, or of different components in amulti-component Galactic model, requires knowledge of the underlying timescaledistribution for each model or component.Comment: 16 pages, latex, + 2 postscript figures. Submitted to ApJ. Fig. 2  replaced, affecting some of the conclusion

On the Robustness of the Microlensing Optical Depth as a Measure of MACHO Density