Abstract
DURING the course of an investigation on the variation with wave-length λ of the stress-optic coefficient Cλ of 'Catalin 800' (a glass-clear phenolformaldehyde resin) and of CR 39 (a condensed allyl ester), it has been found that Cλ depends upon the time allowed to elapse between the application of the load and the measurement of the resultant birefringence, the effect being due to the so-called photo-elastic creep in the loaded specimens. The two specimens, one of 'Catalin' and the other of CR 39, were both strained by a tensile force applied by a simple loading machine working on the lever principle, and the stress-optic coefficients were determined by the modified form of Filon's method1, using a spectrograph instead of the usual spectrometer.
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