\begin{table}%t2
\caption{The $(n, k)$ values obtained by previous authors and in
our present work, for comet Halley at  different wavelengths.}\label{tab2}
%\centering
\par
\begin{tabular}{c c c c}
\hline
\hline
\bf $\lambda$ & $n$ & $k$ & Authors\\
\hline
0.365 $\mu{\rm m}$ & 1.392 & 0.024 & Mukai et~al. (\cite{Muk87})  \\
 & 1.387& 0.032&  Sen et~al. (\cite{Sen91a})\\
 & 1.403& 0.024 & Present work \\
0.484 $\mu{\rm m}$ &1.387 &0.031 & Mukai et~al. (\cite{Muk87})\\
 & 1.375& 0.040&  Sen et~al. (\cite{Sen91a})\\
  &1.390 &0.026 & Present work\\
0.620 $\mu{\rm m}$ & 1.385 &0.035 &  Mukai et~al. (\cite{Muk87})\\
0.684 $\mu{\rm m}$ &1.374 &0.052 & Sen et~al. (\cite{Sen91a})\\
 & 1.386 &0.038 & Present work \\
 \hline
\end{tabular}
\end{table}