Many gravitationally lensed quasars exhibit flux ratio “anomalies”
that cannot be explained under the hypothesis that the lensing
potential is smooth on subgalactic scales. 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 in be used
to infer the stellar fraction of the mass surface density along the
line of sight — typically at impact parameters 1.5 times the half
light radius. We present results from observations of a dozen
multiply imaged quasars observed at the Chandra X-ray Observatory.
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