  The effect of nuclear polarisation on the non-linearities of King plot for
isotope shift in Ca$^+$, Sr$^+$ and Yb$^+$ ions is studied. These ions are
currently used in search for the manifestation of new interaction via
non-linearities of King plot. The role of relativistic effects is also studied.
The best way to achieve separation of electronic and nuclear variables (which
is necessary for the linearity) is to present the field shift as $F"\delta
\langle r^{\lambda} \rangle$, where the value of $\lambda$ is the result of
numerical relativistic calculations ($2\gamma < \lambda <2$, $\gamma =
\sqrt{1-(Z/137.036)^2}$). Using this ansatz we demonstrated that the
contribution of the relativistic effects to the non-linearity in spherical
nuclei is very small.

Atomic Physics