Measurements of the optic index of refraction, along thecaxis, are presented for three representative single crystals in the (PbxBa1−x) Nb2O6ferroelectric system. In particular, we show that as the temperature is lowered, crystals at the morphotropic phase boundary (x=0.63±0.03) first develop a polarization along thecaxis and then, at lower temperature, the polarization rotates to theabplane.