A Study of Gravitational Lens Chromaticity using Ground-based Narrow
  Band Photometry
  We present observations of wavelength-dependent flux ratios for four
gravitational lens systems (SDSS~J1650+4251, HE~0435$-$1223, FBQ 0951+2635, and
Q~0142$-$100) obtained with the Nordic Optical Telescope. The use of narrowband
photometry, as well as the excellent seeing conditions during the observations,
allows us to study their chromatic behavior. For SDSS~J1650+4251, we determine
the extinction curve of the dust in the $z_L=0.58$ lens galaxy and find that
the 2175 \AA \ feature is absent. In the case of HE~0435$-$1223, we clearly
detect chromatic microlensing. This allows us to estimate the
wavelength-dependent size of the accretion disk. We find an R-band disk size of
$r^{R}_s=13\pm5$ light days for a linear prior on $r^{R}_s$ and $r^{R}_s=7\pm6$
light days for a logarithmic prior. For a power-law size-wavelength scaling of
$r_s\propto\lambda^{p}$, we were able to constrain the value of the exponent to
$p=1.3\pm0.3$ for both $r^{R}_s$ priors, which is in agreement with the
temperature profiles of simple thin disk models ($p=4/3$).
