  In gravitational microlensing, interference visibility is established when
the spectral resolution of the observation is less than the reciprocal of the
time delay difference. Using high resolutionspectroscopy fringes will be
visible at large path differences, even when the source emission is incoherent.
This is possible because coherence can be established after interference has
already occurred. Results of a laboratory test are presented which demonstrate
this retroactive establishment of coherence.
