A rheometer for generating uniaxial elongations in molten polymers has been modified to allow for the simultaneous measurement of stress and flow-induced birefringence. Tensile stress sigma and birefringence Delta n＇ data in flows at constant strain rates up to 1 s(-1) were collected on a polydisperse polystyrene melt at temperatures of 160 and 170 degrees C. From these data, the stress-optic rule was followed for stresses below roughly 1 MPa. For stresses less than 1 MPa, the stress-optic coefficient \C\ = \Delta n＇\l sigma was found to have a value of 4.8x10(-9) Pa-1 which was independent of strain, strain rate, and temperature. At stress levels higher than 1 MPa, \C\ decreased indicating a failure of the stress-optic rule. A criteria for failure of the stress-optic rule was formulated using simple arguments from network models and characteristic times suggested by the tube model. This criteria, which is based on the hypothesis that failure of the stress-optic rule is the result of significant chain stretching, was found to be consistent with the data reported in this study and with data from previous studies on polystyrene melts.