68 



THE EQUILIBRIUM OF ELASTIC SOLIDS. 



CASE XIV. 



On the determination of the pressures which act in the interior of trans- 

 parent solids, from observations of the action of the solid on polarized light. 



Sir David Brewster has pointed out the method by which polarized light 

 might be made to indicate the strains in elastic solids ; and his experiments on 

 bent glass confirm the theories of the bending of beams. 



The phenomena of heated and unannealed glass are of a much more complex 

 nature, and they cannot be predicted and explained without a knowledge of the 

 laws of cooling and solidification, combined with those of elastic equilibrium. 



In Case X. I have given an example of the inverse problem, in the case 

 of a cylinder in which the action on light followed a simple law ; and I now 

 go on to describe the method of determining the pressures in a general case, 

 applying it to the case of a triangle of unannealed plate-glass. 



D n 



The lines of equal intensity of the action on light are seen without 

 interruption, by using circularly polarized light. They are represented in Fig. 2, 

 where A, BBS, DDD are the neutral points, or points of no action on light, 

 and CCC, EEE are the points where that action is greatest ; and the intensity 



