We report accurate temperature-dependent measurements of the optic index of refraction, the birefringence, and the strain in the ferroelectric, tungsten-bronze crystal Ba0.4Sr0.6Nb2O6. This composition congruently melts, so large homogeneous high-quality crystals can be grown. From the experimental data, it appears that far above the ferroelectric phase transition temperature (Tc 88°C), up to a temperature Td (300°C), the crystals appear to possess a local, randomly oriented (up or down), polarization Pd. The values of Td obtained from the index and completely independent strain measurements are in very good agreement with each other, as are the values of Pd. Various aspects of our understanding of the polarization behavior and other effects in this ferroelectric system are discussed. © 1987 The American Physical Society.