  Interferometers from the ground and space will be able to resolve the two
images in a microlensing event. This will at least partially lift the inherent
degeneracy between physical parameters in microlensing events. To increase the
signal-to-noise ratio, intrinsically bright events with large magnifications
will be preferentially selected as targets. These events may be influenced by
finite source size effects both photometrically and astrometrically. Using
observed finite source size events as examples, we show that the fringe
visibility can be affected by 5% - 10%, and the closure phase by a few degrees:
readily detectable by ground and space interferometers. Such detections will
offer unique information about the lens-source trajectory relative to the
baseline of the interferometers. Combined with photometric finite source size
effects, interferometry offers a way to measure the angular sizes of the source
and the Einstein radius accurately.
  Limb-darkening changes the visibility by a small amount compared with a
source with uniform surface brightness, marginally detectable with ground-based
instruments.
  We discuss the implications of our results for the plans to make
interferometric observations of future microlensing events.
