We use Sloan Digital Sky Survey Data Release 2 QSO spectra to constrain the dust-reddening caused by intervening damped Ly \alpha systems ( DLAs ) .
Comparing the spectral index distribution of a 70 sight-line DLA sample with that of a large control sample reveals no evidence for dust-reddening at z \sim 3 .
Our limit on the shift in spectral index , \left| \Delta \alpha \right| < 0.19 ( 3 \sigma ) , corresponds to a limit on the colour excess due to SMC-like dust-reddening , \hbox { $E ( B - V ) $ } < 0.02 { mag } ( 3 \sigma ) .
This is inconsistent with the early studies of Fall , Pei and collaborators who used the small QSO and DLA samples available at the time .
Comparison of the DLA and control magnitude distributions also reveals \ga 2 \sigma evidence for an excess of bright and/or a deficit of faint QSOs with foreground DLAs .
Higher equivalent width DLAs give a stronger signal .
We interpret this as the signature of gravitational magnification due to the intervening DLAs .