  Bohr's model of the hydrogen atom can be extended to account for the observed
spin-orbit interaction, either with the introduction of the Thomas precession,
or with the stipulation that, during a spin-flip transition, the orbital radius
remains intact. In other words, if there is a desire to extend Bohr's model to
accommodate the spin of the electron, then experimental observations mandate
the existence of the Thomas precession, which is a questionable hypothesis, or
the existence of artificially robust orbits during spin-flip transitions. This
is tantamount to admitting that Bohr's model, which is a poor man's way of
understanding the hydrogen atom, is of limited value, and that one should
really rely on Dirac's equation for the physical meaning of spin, for the
mechanism that gives rise to the gyromagnetic coefficient g=2, for Zeeman
splitting, for relativistic corrections to Schr\"odinger's equation, for
Darwin's term, and for the correct 1/2 factor in the spin-orbit coupling
energy.
