\begin{table}%t2
%\centering
\par
\caption{\label{tab:historical_plates}Detection limits on plates taken before 1950.}
\begin{tabular}{lllc}
\hline\hline\noalign{\smallskip}
Telescope & Plate  &  Date      &  $m_{\rm pg,~ lim}$\\
          &        & yyyy-mm-dd & \\
\hline\noalign{\smallskip}
Bruce astrograph LHK & 23a   & 1900-09-14 & 18.8\\
Bruce astrograph LHK & 265a  & 1901-08-18 & 18.0\\
Yerkes 24~inch reflector & ? & 1901-09-18 & 19.5\\
Bruce astrograph LHK & 649a  & 1903-01-15 & 18.0\\
Bruce astrograph LHK & 842a  & 1903-09-27 & 18.0\\
Bruce astrograph LHK & 1384a & 1905-12-26 & 18.0\\
Waltz reflector LHK & 198   & 1907-11-02 & 17.5\\
Waltz reflector LHK & 369   & 1908-08-20 & 18.3\\
Waltz reflector LHK & 603   & 1909-10-19 & 18.5\\
Waltz reflector LHK & 4484  & 1934-09-17 & 18.5\\
Bruce astrograph LHK & 7163a & 1949-09-20 & 18.2\\
\hline
&\\
\end{tabular}\\
\end{table}