\begin{table}%T3
%\centering
\par
\caption{Reference clusters, respective metallicities
(according to Harris 1996), and $\Delta(B-V)$ corresponding
to the reddening of NGC~6522 derived differentially relative to the
comparison clusters.
}
\begin{tabular}{lcc}%{lllrrr}
\hline
\hline
\noalign{\smallskip}
{\rm Comparison cluster} & [Fe/H] & {$\Delta(B-V)$} \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
        M 13   &                --1.54  &           0.60 \\
        NGC~6229   &                --1.43   &           0.50 \\
        NGC~6723   &                --1.12  &           0.50 \\
        NGC~6681   &                --1.51  &           0.60 \\
        NGC~6266   &                --1.29  &           0.42 \\
        NGC~5904   &                --1.27  &           0.50 \\
        NGC~1851   &                --1.22  &           0.47 \\
        NGC~1904   &                --1.57  &           0.55 \\
 \hline \end{tabular}
\label{tabredd}
\end{table}