The magnetization is restricted to the plane parallel to the interfaces and also to the plane of incidence. In a special and interesting case, of the axes may be fixed perpendicular to the interfaces while the second is normal to the plane of incidence. Magnetically soft multilayer systems are easily magnetized in the longitudinal direction, which makes the configuration attractive for the in situ magnetooptic measurements. Small electromagnets, which can be put directly into a vacuum chamber, are often sufficient. This configuration is encountered in magnetic multilayers with zero or weak perpendicular magnetic anisotropy. The approximation consists of retaining only the terms independent of magnetization and the terms linearly dependent on the magnetization either in the film or in the substrate.