\begin{table}%t3
\caption{Line ratios and references for the atomic data.}
\label{diagnostic}
%\centerline
{\begin{tabular}{llll}
\noalign{\smallskip}
\hline
\hline
\noalign{\smallskip}
Line ratios    &  T    & C    &   E \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
$[\mbox{\ion{O}{iii}}](\lambda~4959 + \lambda~5007)/\lambda~4363$ &  [6]	& [2]	  & [1,8] \\
$[\mbox{\ion{N}{ii}}](\lambda~6548 + \lambda~6583)/\lambda~5755$  &  [6]	& [2]	  & [1,9] \\
$[\mbox{\ion{S}{ii}}]\lambda~6716/\lambda~6731$                  &  [7,10] & [3]	  & [1,10]\\
$[\mbox{\ion{Cl}{iii}}]\lambda~5517/\lambda~5537$                &  [11,12]& [4]	  & [1]   \\
$[\mbox{\ion{Ar}{iv}}]\lambda~4711/\lambda~4740$                 &  [12,13]& [5]	  & [1]   \\   
\hline
\end{tabular}} \medskip
%\begin{minipage}
{References:} [1] \cite{bowen60}; [2] \cite{lennon94}; [3]~\cite{ramsbottom96}; [4] \cite{butler89}; [5]~\cite{zeippen87}, 
[6] \cite{wiese96}; [7] \cite{keenan93}; [8] \cite{moore85}; [9] \cite{williams95}; [10]~\cite{verner96}; [11] \cite{mendoza83};
[12] \cite{kaufman86}; [13] \cite{mendoza82}.
%\end{minipage}
\end{table}