  We consider the feasibility of directly observing gravitational microlensing
in extra-galactic sources, whose stars are not generally resolved. This
precludes use of the simple optical depth to microlensing formulation, which is
applicable only to resolved stars. We, instead, extend this method to consider
observational constraints, such as seeing, sky background and minimum
detectable change in magnitude. Our analysis provides quantitative relations
between these constraints and the expected observational results, event
duration and number of detections per year. We find that an ambitious
ground-based observer should detect several events per year in M31. We also
consider detection of microlensing in visual binary galaxies. We find that,
although these objects may present hundreds of events per year, extremely short
durations would yield poor prospects for observation.
