The correction was predicted in 1963 by Magda Ericson and was derived in 1966 together with Torleif Ericson. The effect has since been re-derived in various ways, but is now understood as a general effect as long as the nucleons keep their individuality, independent of the detailed cause. This is the reason why in the molecular case of the classical Lorentz-Lorenz effect so many incompatible derivations give the same result. The EELL correction was first applied to the line shifts of hydrogen-like atoms, where the electron in the Coulomb field is replaced by a negatively charged pion. Its interaction with the central nucleus causes deviations in the line positions in such Bohr-like atoms.