The Paschen-Bach effect.

Every single line of a multiplet exhibits in a weak magnetic field a typical Zeeman effect whose separations are proportional to the field and are symmetrical with respect to the original line. (The field is regarded as weak when the Zeeman separations are of smaller order than the width of the original multiplet.) If the field is increased, the Zeeman lines of the different components of the multiplet spread out and overlap. Before this overlapping begins, an unsymmetrical distortion of the single Zeeman types shows itself, and this applies to the intensities as well as to the lie of the lines when the field becomes strong; certain lines flow together while others, losing their intensity, vanish; and finally the lines form a normal Zeeman triplet whose separation is large compared with the breadth of the original multiplet. This normalising of the Zeeman effect by strong fields was discovered by Paschen and Back.

General theory of the Paschen-Back effect.

The model of a multiplet state has coupled momenta S, L whose resultant is J, precessing round J. If the magnetic field is weak and so does not affect the internal coupling between S and L, its effect is to cause a precession of J about the axis of the field.