  If cluster dark matter is in the form of compact objects it will introduce
fluctuations into the light curves of distant sources. Current searches for
MACHOs in clusters of galaxies focus on monitoring quasars behind nearby
systems. This paper considers the effect of such a compact population on the
surface brightness distribution of giant gravitationally lensed arcs. As the
microlensing optical depth is significant in these clusters, the expected
fluctuations are substantial and are observable. Focusing on the giant arc seen
in Abell 370, we demonstrate that several `extreme' events would be visible in
a comparison of HST observations at two epochs. Utilizing NGST, long term
monitoring should reveal a ubiquitous twinkling of brightness over the surface
of the arcs.
