On the basis of Lorentz's theory, if we limit ourselves to a single spectral line, it suffices to assume that each atom (or molecule) contains a single moving electron.

Now all oscillatory movements of such an electron can be conceived of as being split up into force, and two circular oscillations perpendicular to this direction rotating in opposite directions.

Now if this electron is displaced from its equilibrium position, a force that is directly proportional to the displacement restores it like a pendulum to its position of rest.

Pieter Zeeman Quotes