We report measurements of the temperature dependence of the optic index of refraction, n(T), at several wave lengths in two ferroelectric compounds that have the simple perovskite ABO3 structure. The compounds are Pb(Mg 1 3Nb 2 3)O3 and Pb(Zn 1 3Nb 2 3)O3 where the B-site ions are disordered. Deviations from the high temperature linear n(T) can be observed starting at a temperature Td (far above the ferroelectric phase transition temperature Tc). We assume a model in which these deviations arise from very local randomly oriented polarization (i.e. glassy polarization), Pd. Then, using the quadratic electrooptic effect and the known coefficients, we determine Pd and compare it to the normally observed ferroelectric reversible spontaneous polarization Pr. Also, from infrared reflectivity data, we show that these materials are microscopically homogeneous. We believe that our model of the phase transition contains the basic physics describing ferroelectrics with diffuse phase transitions. © 1983.