The change in length of a Vinylite plastic under constant load has been measured, as well as for the succeeding recovery. The change in color of the light transmitted through crossed Polaroid plates shows large change in the birefringence. The further study is made with uniform length increments at constant temperature using Mach's method to measure the birefringence, both for stretching and recovery. The data are sufficient to give a measure of the dispersion in birefringence and of a Poisson's ratio of 0.50.