\begin{table}%t5
\caption{Properties of the samples of \ha\ candidates.\label{selection}} 
%\centerline
{\begin{tabular}{c c c c c c c} \hline \hline
$EW_0$ & $\Sigma$ & $m_{\rm NB}$ & \# & \#H & $n_{1138}$ & $n$ \\
(1) &(2)&(3) &(4)&         (5) &  (6) &  (7) \\ \hline
 50 & 3 & --   & 17 & 2 & $1.4\pm0.3$ \\
 25 & 3 & --   & 23 & 3 & $1.8\pm0.4$ \\
 25 & 2 & --   & 40 & 3 & $3.2\pm0.5$ \\
 50 & 1 & --   & 48 & 2 & $3.8\pm0.6$ \\
 25 & 1 & --   & 60 & 3 & $4.8\pm0.6$ \\
 75 & 2 & 19.3 &  1 & 0 & $0.1\pm0.1$ & $0.61\pm0.35^a$ \\
100 & 3 & 19.5 &  2 & 1 & $0.2\pm0.1$ & $0.12\pm0.05^b$ \\
\hline  
\end{tabular}}
\smallskip
\noindent Notes: (1) Rest frame equivalent width lower
limit; (2) signal to noise lower limit as defined by \citet{bun95};
(3) additional selection criterion: narrow band magnitude upper limit;
(4) number of candidates in 12.5 arcmin$^2$ area; (5) number of
candidates within the radio galaxy \lya\ halo; (6) surface density of
candidates in the field of \rgs\ (arcmin$^{-2}$); (7) surface density
in other fields: $^a$ \citet{bun95}, $^b$ \citet{wer00}.
%%\end{center}
\par
\end{table}