Simultaneous measurements of the gyration, birefringence, and rotation angle of the optical indicatrix of Rochelle salt were successfully performed by using high-accuracy universal polarimetry. The three phases of the crystal were found to be optically active. It was revealed that the origin of ferroelectricity of the crystal is directly responsible for the occurrence of the optical activity. Birefringence in the ferroelectric phase cannot be explained by the usual electro-optic effect. Two possibilities are considered for this discrepancy.
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