  The ratio $R_T$ of $K^+-^{12}C$ to $K^+-d$ cross sections has been calculated
microscopically using a boson-exchange $KN$ amplitude in which the $\sigma$ and
$\omega$ mesons are dressed by the modifications of the Dirac sea in nuclear
matter. In spite of the fact that this dressing leads to a scaling of the
mesons effective mass in nuclear matter, the effect on the $R_T$ ratio is found
to be weak.
