A study was made of the effect of hydrostatic pressure upon the relaxation of birefringence in two commercial plastics which were glasslike in nature. The birefringence was introduced into the materials by loading in uniaxial compression, and the decay of birefringence with time was measured using a polarimeter. The rate at which the optical path-difference disappeared was greatly inhibited by the hydrostatic pressure, and a pressure of 10,000 bars was found to be sufficient to stop the release completely. The results are interpreted as the effect of hydrostatic pressure upon the mobility of the materials. Some experiments were conducted to determine the effect of temperature upon the relaxation, that the effects of pressure and temperature might be compared.
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