\begin{table}%t5
\caption{\label{oiirat}[\ion{O}{ii}] $\lambda3729/\lambda3726$ intensity ratios 
of low-$N_{\rm e}$ plasmas from literature. }
%\centerline
{
\begin{tabular}{lccc}
\hline
\hline
Source & Obs. Pos & $I(\lambda3729)/I(\lambda3726)$ & Ref. \\
\hline
 IC~418      & 180$^{\prime\prime} $  & $\rm 1.79^{2.83}_{1.30}$ &[1]\\
%&[1]\\
 PX Pup     & 14$^{\prime\prime}$    & $\rm 1.43^{1.54}_{1.33}$ &[1]\\
 He 2-146   &        & $\rm 1.43^{1.45}_{1.41}$ &[2]\\
 NGC 7293   &        & $\rm 1.41^{1.43}_{1.39}$ &[2]\\
 IC 5148-50 &        & $\rm 1.45^{1.49}_{1.41}$ &[2]\\
 NGC 6543    &Halo    & $\rm 1.44^{1.56}_{1.32}$ &[3]\\
 NGC 2371-2 & 9$^{\prime\prime}$     & $\rm 1.47^{1.73}_{1.21}$& [4]\\  
%&[4]\\            
\hline
\end{tabular}}
\smallskip
  References:  
 [1]~Monk et~al. (\cite{MBC1990}),
 [2]~Kingsburgh \& English (\cite{kingsburgh1992}),
 [3]~Middlemass et~al. (\cite{MCW1989}),
 [4]~Kingsburgh \& Barlow (\cite{KB92}).
\end{table}