Top panel: effective optical depth of the dimming layer vs. wavelength computed from the ratio of the dim to the bright X-shooter spectra. The red circles are the median of the values over intervals of 5 Å, the error is the standard deviation within the interval. The triangles show the optical depth derived from the ratio of the CObandheads, computed as the median of three points around the peak of the first two bandheads. The black crosses correspond to continuum points in the spectrum. Bottom panel: the red points (circles and triangles) are the same as in the top panel. The black dashed line is the Cardelli extinction law for R_V = 3.1 (Cardelli et al. 1989). The green curves plot Q_abs (dashed), Q_scat (dotted), and Q_tot = Q_abs + 0.12Q_scat (solid) for a distribution of olivine with minimum and maximum size a_min = 0.1 μm, a_max = 150 μm, q = 3 (see text). The blue curve plot Q_tot = Q_abs + Q_scat (solid) for the same material, a_min = 0.06 μm, a_max = 0.1 μm, q = 3. All curves are normalised to the observed optical depth at 0.58 μm.