We report the detection of differential gas column densities in three gravitational lenses , MG 0414+0534 , HE 1104 - 1805 , and PKS 1830 - 211 .
Combined with the previous differential column density measurements in B 1600+434 and Q 2237+0305 and the differential extinction measurements of these lenses , we probe the dust-to-gas ratio of a small sample of cosmologically distant normal galaxies .
We obtain an average dust-to-gas ratio of E ( B - V ) / { N } _ { H } = ( 1.4 \pm 0.5 ) \times 10 ^ { -22 } mag cm ^ { 2 } atoms ^ { -1 } with an estimated intrinsic dispersion in the ratio of \simeq 40 % .
This average dust-to-gas ratio is consistent with the average Galactic value of 1.7 \times 10 ^ { -22 } mag cm ^ { 2 } atoms ^ { -1 } and the estimated intrinsic dispersion is also consistent with the 30 % observed in the Galaxy .