One correction to the Dirac theory involves the use of the reduced electron
mass, which was previously discussed. Another involves considering that
the proton has some finite size and is not exactly a point charge. Instead
of having a 
potential energy, one might imagine something like
where 
is some representative size of the proton 
cm.
Like the Darwin term, this will only affect s states, since only in these
states can the electron be found at the origin. However, even for s
states this correction turns out to be of the order
, and hence it is not very important.
Other effects will now be discussed which are more important, not only
because the energy shifts are larger, but because they split the energy
levels of states that would otherwise be degenerate.