Very large rotations of the axis of linear polarization may be observed in visible light passing through ferromagnetic chromium trihalides. The greatest effects are apparently associated with an absorption band edge which, for CrBr 3, is located in the green. This paper presents the spectral variation of that rotation and of the corresponding absorption band edge. We consider a model consisting of a Cr 3+ ion in an octahedron of Br − ions, subjected to a molecular field. It is shown that the absorption band edge in CrBr 3 corresponds to charge transfer transitions in which an electron in a bromine orbital is promoted into an obital localized on the central chromium ion. The rotation arises as a consequence of the splitting of the excited energy levels by the spin-orbit coupling and the trigonal crystal field.