\begin{table}%t1
%\begin{center}
\par
\caption{The number of individual $\Delta a$ measurements from the given reference. The used
filter systems are according to Table \ref{filters}.}
%\vspace*{2mm}
\label{n_data}
\begin{tabular}{lrc}
\hline
\hline
Ref. & $N_{\rm obs.}$ & System \\
\hline
Maitzen (\cite{Maitzen76}) & 168 & 1 \\
Maitzen (\cite{Maitzen80a}) & 10 & 2 \\
Maitzen (\cite{Maitzen80b}) & 8 & 2 \\
Maitzen \& Seggewiss (\cite{Maitzen80}) & 21 & 2 \\ 
Maitzen (\cite{Maitzen81}) & 20 & 2 \\ 
Maitzen \& Vogt (\cite{Maitzen83}) & 342 & 3 \\
Maitzen \& Pavlovski (\cite{Maitzen89a}) & 16 & 4 \\ 
Maitzen \& Pavlovski (\cite{Maitzen89b}) & 31 & 4 \\
Pavlovski \& Maitzen (\cite{Pavlovski89}) & 40 & 4 \\
Schnell \& Maitzen (\cite{Schnell94}) & 3 & 5 \\
Schnell \& Maitzen (\cite{Schnell95}) & 14 & 5 \\
Maitzen et~al. (\cite{Maitzen97}) & 6 & 6 \\
Maitzen et~al. (\cite{Maitzen98}) & 131 & 7 \\
Vogt et~al. (\cite{Vogt98}) & 803 & 3 \\
\hline
\end{tabular}
%\end{center}
\end{table}