Context : Observations of quasars shining through foreground galaxies , offer a way to probe the dust extinction curves of distant galaxies .
Interesting objects for this study are found in strong gravitational lensing systems , where the foreground galaxies generate multiple images .

Aims : The reddening law of lensing galaxies is investigated by studying the colours of gravitationally-lensed quasars , and a handful of other quasars where a foreground galaxy is detected .

Methods : We compare the observed colours of quasars reported in the literature , with spectral templates reddened by different extinction laws and dust properties .
The data consists of 21 quasar-galaxy systems , with a total of 48 images .
The galaxies , which are both early- and late-type , have redshifts in the interval z = 0.04 - 1.51 .

Results : We measure a difference in rest-frame B - V between the quasar images we study , and quasars without resolved foreground galaxies .
This difference in colour is indicative of significant dust extinction in the intervening galaxy .
Good fits to standard extinction laws were found for 22 of the images , corresponding to 13 different galaxies .
Our fits imply a wide range of possible values for the total-to-selective extinction ratio , R _ { V } .
The distribution was found to be broad with a weighted mode of \bar { R } _ { V } =2.4 and a FWHM of \Delta { R { { } _ { V } } } =2.7 ( \sigma _ { R _ { V } } \sim 1.1 ) .
Thus the bulk of the galaxies for which good reddening fits could be derived , have dust properties compatible with the Milky Way value ( R _ { V } = 3.1 ) .

Conclusions :