A Study of Gravitational Lens Chromaticity with the Hubble Space
  Telescope
  We report Hubble Space Telescope observations of 6 gravitational lenses with
the Advanced Camera for Surveys. We measured the flux ratios between the lensed
images in 6 filters from 8140\AA\ to 2200\AA. In 3 of the systems,
HE0512$-$3329, B1600+434, and H1413+117, we were able to construct UV
extinction curves partially overlapping the 2175\AA\ feature and characterize
the properties of the dust relative to the Galaxy and the Magellanic Clouds. In
HE1104$-$1804 we detect chromatic microlensing and use it to study the physical
properties of the quasar accretion disk. For a Gaussian model of the disk
$\exp(-r^2/2 r_s^2)$, scaling with wavelength as $r_s \propto \lambda^p$, we
estimate $r_s(\lambda3363)=4^{+4}_{-2}$ ($7\pm 4$) light-days and $p=1.1\pm
0.6$ ($1.0\pm 0.6$) for a logarithmic (linear) prior on $r_s$. The remaining
two systems, FBQ0951+2635 and SBS1520+530, yielded no useful estimates of
extinction or chromatic microlensing.
