\begin{table}%t1
\par
\caption {Sources of data for continuous opacities.\label{tab1}}
%\centerline
 {\begin{tabular}{l l}
\hline\hline
Ion and process  & Reference \\
\hline
H~{\sc i} b-f$^1$, f-f$^1$ & Karzas \& Latter (\cite{Karzas61})$^\ddagger$ \\
H$^-$ b-f & Wishart (\cite{Wishart79}) \\
H$^-$ f-f & Bell \& Berrington (\cite{Bell87}) \\
He~{\sc i} b-f & TOPbase$^2$ \\
He~{\sc i} f-f & Peach (\cite{Peach70}) \\
He$^-$ f-f & John (\cite{John94}) \\
C~{\sc i}, C~{\sc ii} b-f & TOPbase$^\dagger$ \\
C~{\sc i}, C~{\sc ii} f-f & Peach (\cite{Peach70}) \\
C$^-$ f-f & Bell et~al. (\cite{Bell88}) \\
N~{\sc i}, N~{\sc ii} b-f & TOPbase \\
N$^-$ f-f & Ramsbottom et~al. (\cite{Ramsbottom92}) \\
O~{\sc i}, O~{\sc ii} b-f & TOPbase \\
O$^-$ f-f & John (\cite{John75}a,b) \\
Mg~{\sc i}, Mg~{\sc ii} b-f & TOPbase$^\dagger$ \\
Mg~{\sc i} f-f & Peach (\cite{Peach70}) \\
Al~{\sc i}, Al~{\sc ii} b-f & TOPbase$^\dagger$ \\
Si~{\sc i}, Si~{\sc ii} b-f & TOPbase$^\dagger$ \\
Si~{\sc i} f-f & Peach (\cite{Peach70}) \\
Ca~{\sc i}, Ca~{\sc ii} b-f & TOPbase \\
Fe~{\sc i} b-f & TOPbase, Bautista (\cite{Bautista97}) \\
Fe~{\sc ii} b-f & TOPbase \\
other metals f-f & Peach (\cite{Peach70}), hydr. approx.\\
H$_2^+$ f-f & Mihalas (\cite{Mihalas64}) \\
H$_2^-$ f-f & John (\cite{John75}a,b), John \& Williams (\cite{John75}) \\
CH b-f & Kurucz et~al. (\cite{Kurucz87}) \\
OH b-f & Kurucz et~al. (\cite{Kurucz87}) \\
CO$^-$ f-f & John (\cite{John75}a,b) \\
H$_2$O$^-$ f-f & John (\cite{John75}a,b) \\
H~{\sc i}+H~{\sc i} CIA$^3$ & Doyle (\cite{Doyle68}) \\
H~{\sc i}+He~{\sc i} CIA & Gustafsson \& Frommhold (\cite{Gustafsson01}) \\
H$_2$+H~{\sc i} CIA & Gustafsson \& Frommhold (\cite{Gustafsson03}) \\
H$_2$+H$_2$ CIA & Borysow et~al. (\cite{Borysow01}) \\
H$_2$+He~{\sc i} CIA & J\o rgensen et~al. (\cite{Jorgensen00})\\
H~{\sc i} scattering & Dalgarno, quoted by Gingerich (\cite{Gingerich64})\\
H$_2$ scattering & Dalgarno \& Williams (\cite{Dalgarno62})\\
e$^-$ scattering & Mihalas (\cite{Mihalas78})\\
He~{\sc i} scattering & Bues \& Wehrse (\cite{Bues76})\\
\hline
\end{tabular}}
\smallskip
{$^1$ ``b-f'' and ``f-f'' denote bound-free and free-free} 
{processes, respectively.} \\
{$^2$ ``TOPbase'': data from the Opacity Project (Seaton et~al. \cite{Seaton94}), } \\
{available via \url{http://vizier.u-strasbg.fr/topbase/home.html}.} \\
{$^3$ ``CIA'': collision induced or quasi-molecular absorption.}  \\
{$^\ddagger$~Modified using the occupation probability formalism, see text.}  \\
{$^\dagger$ The calculated wavelengths of UV and blue C~{\sc i}, Mg~{\sc i},}  
{Al~{\sc i} and Si~{\sc i} absorption edges have been shifted to their.}  
{laboratory values, see text.}
\par
\end{table}