The photo-elastic constants of glass as affected by high temperatures and by lapse of time

(a ) In 1914, Prof. L. N. G. Filon carried out some experiments on the effect of temperature on the photo-elastic constants of glass, using the method of flexure, and enclosing the specimen in a steam jacket giving a maximum temperature of 90°C. He observed a small increase in the stress-optical coefficient for the ray polarized perpendicularly to the line of stress. The following series of experiments were carried out with the object of extending the observations for temperatures up to near the melting-point. The specimens consisted of small blocks of glass—4 × 3 × 1 cms. A small electric furnace was constructed of “asbestone,” into which the specimen was placed and subjected to simple pressure. To enable the pressure to be applied freely to the specimen whilst in the furnace, a sheet of thin, pliable asbestos (AB, fig. 1) was laid over the specimen between it and the upper knife-edge block and extending to the outer walls of the furnace, being supported by the inner walls. Over this pliable sheet was spread a layer of asbestos wool. The knife-edge block was further separated from the specimen by a layer of asbestos about ½ cm. thick Fig. 1 shows the arrangement of the top knife-edge on the specimen. The whole furnace, with specimen enclosed, was supported on the lower knife-edge. In this way the specimen could be maintained .at a uniform temperature whilst under pressure.