\begin{table}%t4
\renewcommand{\arraystretch}{1.25}
\caption{\label{tab4}The correlations between $V-I$ colour index and $R$~magnitude, for the eight blazar objects.}
%\centering
\par
\begin{tabular}{lcccc}
\hline \hline
Source & $\Delta R$ & $N$ & $r$ & Probs.\\
\hline
\par
3C 66A          &  0.50  &  120  &   0.511 & $7.34 \times10^{-10}$ \\
PKS 0420$-$01  &  1.48  &  55   &  --0.388 & $2.62 \times10^{-3}$  \\
S5 0716+71     &  1.37  &  271  &   0.438 &  $1.11 \times10^{-14}$ \\
PKS 0735+17    &  0.54  &  38   &   0.111 &  $4.97 \times10^{-1}$  \\
OJ 287          &  1.40  &  29   &   0.354 & $4.99 \times10^{-2}$  \\
3C 345          &  0.55  &  22   &  --0.009& $9.66 \times10^{-1}$  \\
BL~Lac          &  1.05  &  53   &   0.657 & $1.33 \times10^{-8}$  \\
3C 454.3        &  0.93  &  48   &  --0.837 &$1.07 \times10^{-16}$ \\
\hline
\end{tabular}
\end{table}