this integral does not depend on $\omega $, leading to a constant $\varepsilon _{\left( i\right)
}^{\prime }\left( \omega \right) $. In reality the situation is more complicated because the transition is not sharp and many factors can influence the transition region. We will assume here that ${\cal P}$ is a constant but the fitting procedure will be shifted to frequencies where the transition tail is not very important. In practice Eq. (\[DrudeRI\]) can be applied for $\omega <1$ eV.

Our purpose is now to establish the magnitude of the force change due to reasonable variation of the optical properties. To this end the available low-frequency data for $\varepsilon ^{\prime }\left(
\omega \right) $ and $\varepsilon ^{\prime\prime }\left( \omega
\right) $ presented in the left graph of Fig. \[fig2\] were fitted with Eq. (\[DrudeRI\]). The results together with the expected errors are collected in Table \[tab1\].

  N   $\ \ \ \omega _{p}$(eV)   $\omega _{\tau }\cdot 10^{2}$(eV)       ${\cal P}$     
  --- ------------------------- ----------------------------------- ------------------ --------------------------------------------------------
  1   $7.50\pm 0.02$            $6.1\pm 0.07$                       $-27.67\pm 5.79$   Palik, 66 points , $\ \cdot$
  2   $8.41\pm 0.002$           $2.0\pm 0.005$                      $7.15\pm 0.035$    Weaver, 20 points,  $\blacksquare, \Box $
  3   $8.84\pm 0.03$            $4.2\pm 0.06$                       $12.94\pm 16.81$   Motulevich, 11 points,  $\bullet, \circ$
  