  Microlensing studies of quasars can reveal dark matter lumps over a broad
mass spectrum; we highlight the importance of monitoring quasars which are seen
through the halos of low-redshift galaxies. For these configurations
microlensing by planetary-mass objects will manifest itself as isolated events
which are only weakly chromatic. Statistical comparison of the observed optical
depths with their theoretical counterparts provides a strong test for a
microlensing origin of such events. If microlensing is detected, the
light-curves can reveal not only the characteristic microlens masses, and their
corresponding contribution to dark halos, but also how compact the individual
objects are. In this way we can decisively test the possibility that the dark
matter associated with galaxies is composed principally of planetary-mass gas
clouds.
