  Many gravitationally lensed quasars exhibit flux ratio "anomalies" that
cannot be explained under the hypothesis that the lensing potential is smooth
on scales smaller than one kpc. Micro-lensing by stars is a natural source of
granularity in the lens potential. The character of the expected fluctuations
due to micro-lensing depends sensitively on the relative surface densities of
micro-lenses (stars) and smoothly distributed (dark) matter. Observations of
flux ratios may therefore be used to infer the ratio of stellar to dark matter
along the line of sight -- typically at impact parameters 1.5 times the half
light radius. Several recently discovered systems have anomalies that would
seem to be explained by micro-lensing only by demanding that 70-90% of the
matter along the line of sight be smoothly distributed.
