  If there is a population of black holes distributed randomly in space, light
rays passing in their vicinity will acquire random phases. In the "two-slit"
model of an interferometer this can, for a high density of \bhsn, lead to a
diffusion in the phase difference between the two arms of the interferometer
and thus to a loss of coherence or ``visibility''in interferometric
observations. Hence the existence of "fringe constrast" or "visibility" in
interferometric observations can be used to put a limit on the possible
presence of \bhs along the flight path.
  We give a formula for this effect and consider its application, particularly
for observations in cosmology. Under the assumption that the dark matter
consists of black holes, we consider sources at high z, up to the CMB. While
the strongest results are for the CMB as the most remote source, more nearby
sources at high z lead to similar effects.
  The effect increases with the baseline, and in the limiting case of the CMB
we find that with earth-size baselines a non-zero "visibility" would limit the
mass of possible primordial black holes, to approximately $M/M_\odot \leq
10^{-1}$. Although such limits would not appear to be as strong as those
obtained, say from microlensing, they involve a much different methodology and
are dominated by very early times (see Table 1).
  Longer baselines lead to more stringent limits and in principle with extreme
lengths, the method could possibly find positive evidence for primordial black
holes. In this case, however, all other kinds of phase averaging would have to
be constrained or eliminated.
