\begin{table}
   %\centering
\caption{\label{surveydiffs}Bright (classes 1 to 4) and dark (5--6) peaks
   for the various survey data sources. The columns contain the data source,
   the number of bright and dark peaks, the ratio between dark and bright
   peaks, the average exposure times and image seeing, the area covered, plus
   the number of bright and dark peaks per unit area. For the COMBO-17 field we
   give in parenthesis the corresponding values when the known structures are
   subtracted.} 
  %\centering
 \begin{tabular}{lcccccccc}
   \noalign{\smallskip}
   \noalign{\smallskip}
   \hline 
   \hline 
   \noalign{\smallskip}
   \noalign{\smallskip}
   Source & $n_{\rm b}$ & $n_{\rm d}$ & $n_{\rm d}/n_{\rm b}$ 
   & t[ks] & s[ \myarcsecnodot] & A [$~\Box^2$] & $n_{\rm b}/A$ & 
   $n_{\rm d}/A$ \\ 
   \noalign{\smallskip}
   \noalign{\smallskip}
   \hline 
   \noalign{\smallskip}
   EDisCS      & 10  & 21 & 2.1 & 3.5  & 0.90 & 3.8 & 2.6 & 5.5 \\
   ASTROVIRTEL & 18  & 27 & 1.5 & 4.9  & 0.87 & 6.1 & 3.0 & 4.4 \\
   COMBO-17    & 25 (18) & 13 & 0.5 (0.7) & 25.7 & 0.80 & 1.6 & 15.6 (11.2) & 8.1 \\
   EIS         & 6   & 7  & 1.2 & 6.8  & 0.94 & 2.9 & 2.1 & 2.4 \\
   Own obs.    & 13  & 18 & 1.4 & 8.9  & 0.88 & 4.2 & 3.1 & 4.3 \\
   \hline 
   \end{tabular}
   \normalsize
\end{table}