  This work presents the results for the Faraday rotation in pure and Cr-doped
lead germanate crystals studied in the course of proper ferroelectric phase
transition. We show that the increment of the Faraday rotation appearing at the
phase transition is caused by a combined magneto-electrooptic effect induced by
spontaneous polarization. It is proportional to the square of spontaneous
polarization. The phenomenon revealed by us corresponds to combined effects of
crystal optics, which appear due to common action of different fields.
