The quantum field theoretical formulation of optical birefringence based upon theSmatrix and the Stokes parameter description of polarization is applied in a study of the birefringence of liquids induced by a static magnetic field. The phenomena treated are the Faraday effect (for an arbitrary ellipticity of the incident beam), the Cotton-Mouton effect and the quadratic Faraday effect in which the angle of rotation of the plane of polarization is proportional to the square of the magnetic field strength and which occurs in solutions of naturally optically active molecules.