\begin{table}%t1
\caption{Maximum polarization (\%) for 2- and 10-monomer aggregates.}           
\label{tab1}                               % used for centering table
%\begin{minipage}[t]{18cm}
%\centering
\par
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\begin{tabular}{l c c }        % centered columns (4 columns)
\hline\hline \noalign{\smallskip}               
Composition, wavelength, &10-monomer &2-monomer \\   
and refractive index  &aggregate &aggregate\\
\hline  
Cometary (Halley-like) dust & \\		
  $0.45~\mu {\rm m}$,    $ m=1.88 - i 0.47$ &	59.2 &	56.9 \\
  $0.6~\mu {\rm m}$,    $ m=1.98 - i 0.48$  &	87.2 &	87.0 \\
  $2.2~\mu {\rm m}$,    $ m=2.36$  $-i 0.457$ &	91.7 &	97.2  \\                    
\hline
$m=1.9-i0.5$ & \\		
  $0.45~\mu {\rm m}$ &	58.0 &	55.0 \\
  $0.6~\mu {\rm m}$ &	89.0	& 88.6\\
  $2.2~\mu {\rm m}$ &	94.4	& 98.3\\
\hline
$m=1.5-i0.001$&	&		\\
 $ 0.45~\mu {\rm m}$&	92.9&	94.5\\
  $0.6~\mu {\rm m}$&	96.9&	98.1\\
  $2.2~\mu {\rm m}$&	98.5&	99.5\\
\hline         
\end{tabular}
%\end{minipage}
\end{table}