  We present the results of studies for magnetogyration (MG) effect in
non-centro-symmetric, non-polar Bi12GeO20 crystals performed with the
small-angular polarimetric mapping technique. The magnitude of MG rotation
obtained with the small-angular mapping is close to the corresponding values
obtained recently on the basis of single-ray polarimetry. Our results
demonstrate that multiple reflections of light play a key role in the studies
of MG effect in Bi12GeO20 crystals. They lead to the error, which is at least
two times as large as the expected value of the MG rotation.
