aligned along the slit. Our sample contains two lensed quasars: QSO B0908+0603 (double system) and QSO B1104-181 (quadruple system). In the former case two objects, and, in the latter case, three objects were aligned on the slit [@lopez07]. In these cases we only analyzed the brightest objects. Our sample contains four quasar pairs: QSO J0008-2900 & QSO J0008-2901 (separated by $1.3'$), J030640.8-301032 & J030643.7-301107 (separated by $\sim$0.85$'$), QSO B0307-195A & B (separated by $\sim$1$'$), and QSO J1039-2719 & QSO B1038-2712 (separated by $17.9'$). For these eight objects, eight different slit-positions were used.

The DLA/sub-DLA Sample
======================

The quasar spectra were normalized to unity within the `MIDAS` environment. For each quasar the local continuum was determined in the merged spectrum by using a spline function to smoothly connect the regions free of absorption features (see Fig. \[conti\]). The final normalized spectra used for column density analysis were obtained by dividing merged spectra by these continua. During normalization, the artifact from residual fringes (especially in standard setting centered on $\lambda$ 860nm) and spectral gaps are also included in the spline fitting. In order to secure all DLAs and sub-DLAs in a given spectrum, we proceeded as in @lanzetta91 and used an automated 