Atomic and nuclear physics are two flourishing but distinct branches of
physics; the subject of isotope shifts in atomic spectra is one of the
few that links these two branches. It is a subject that has been studied
for well over fifty years, but interest in the subject, far from
flagging, has been stimulated in recent years. Fast computers have
enabled theoreticians to evaluate the properties of many-electron atoms,
and laser spectroscopy has made it possible to measure isotope shifts in
the previously unmeasurable areas of very rare isotopes, short-lived
radioactive isotopes, weak transitions, and transitions involving
high-lying atomic levels. Isotope shifts can now be measured with
greater accuracy than before in both optical transitions and x-ray
transitions of muonic atoms; this improved accuracy is revealing new
facets of the subject. I am very grateful to Dr. H. G. Kuhn, F. R. S.,
for having introduced me to the subject in the 1950s, and for
supervising my efforts to measure isotope shifts in the spectrum of
ruthenium. I thus approach the subject as an experimental atomic
spectroscopist. This bias is obviously apparent in my use of the
spectroscopist's notation of lower-upper for a transition, rather than
the nuclear physicist's upper-lower. My reasons are given in Section 1.
3 and I hope that nuclear physicists will forgive me for using this
notation even for muonic x-ray transitions.