\begin{table}%f7
%\centering
\par
\caption{ Electron density indicators in NGC~40.}\label{t7}
\begin{tabular}{lcccc}
\hline
\hline
Ion &Ioniz. & Lines& Observed &$N_{{\rm e}}$ \\
&pot. (eV) & used  & ratio & (cm$^{-3}$)\\
\hline
\ion{[S}{ii]} & 10.4 & 6731/6716 & 1.39 & 2100\\
\ion{[O}{ii]} & 13.6 & 3626/3729 & 1.3 & 1400\\
\ion{[N}{ii]} & 14.5 & 6584/122  & 140. & 2000\\
\ion{[S}{iii]} & 23.3 & 33.5/18.7 & 0.76 & 1100\\
\ion{[Cl}{iii]} & 23.8 & 5538/5518 & 1.3: & 4500\\
\ion{[C}{iii]} & 24.4 & 1907/1909 & 1.27 & 4000 \\
\ion{[O}{iii]} & 35.1 & 52/88 & 1.7 & 800\\
\hline
\end{tabular}
\end{table}