\begin{table}%t2
\caption{\label{t2_n}Dependency of $t^2_{\rm s}$ on the number of apertures $N$.}
\small%\centering
\par
\begin{tabular}{lllllll}
\hline
\hline\noalign{\smallskip}
\multicolumn{1}{l}{} & 
\multicolumn{6}{c}{$t^2_{\rm s}(N)/t^2_{\rm s}(N'=100)$ 
(per cent)} \\
$N$ & Bac & \ion{O}{iii} & \ion{N}{ii}~& \ion{O}{i} & \ion{Ar}{iii} & \ion{C}{i} 
\\
\hline
5  & 60--90 & 62--91 & 45--88 & 00--27 & 70--98 & 28--83 \\
10 & 85--97 & 84--96 & 67--92 & 25--56 & 83--99 & 58--87 \\
20 & 97--99 & 94--99 & 84--94 & 41--78 & 92--99 & 79--92 \\
\hline
\end{tabular}
\end{table}