We present observations of wavelength-dependent flux ratios for four gravitational lens systems ( SDSSÂ J1650+4251 , HEÂ 0435 - 1223 , FBQ 0951+2635 , and QÂ 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 SDSSÂ J1650+4251 , we determine the extinction curve of the dust in the z _ { L } = 0.58 lens galaxy and find that the 2175 Ã Â feature is absent .
In the case of HEÂ 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 \pm 5 light days for a linear prior on r ^ { R } _ { s } and r ^ { R } _ { s } = 7 \pm 6 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 \pm 0.3 for both r ^ { R } _ { s } priors , which is in agreement with the temperature profiles of simple thin disk models ( p = 4 / 3 ) .