574 Professor Ernest G. Coker [Feb. 18, 



stressed become doubly refractive, should not have been more 

 frequently pressed into service, for its use was immediately obvious 

 to the discoverer, who pointed out that the stresses in the arched 

 rings of bridges, could be rendered visible in a glass model by aid of 

 the doubly refractive effect produced by a beam of polarized light. 



Here and there one finds accounts of applications of this property 

 for engineering work, but usually with little success, mainly no 

 doubt, to the difficulties experienced in shaping glass models to the 

 required form ;- but when these are overcome the value of the infor- 

 mation gained is very great, as, for instance, in the recent valuable 

 investigations of the stresses made upon a glass model of a reinforced 

 concrete arch by M. Mesnager, of Paris, who used the results so 

 obtained for the design of an arch of about 310 feet span, with a 

 most gratifying agreement between the stresses in the actual bridge 

 and its model. The expense and difficulty of constructing glass 

 models is a bar to their general use, and other transparent materials 

 are now available which offer many advantages, in that they are 



Fig. 1, 



strongly doubly refracting under stress, are easily fashioned with 

 engineering tools, and are not readily broken or damaged, while the 

 cost is insignificant. 



Here, for example, is a rough model of an arch-ring, and you 

 observe that when loads are applied, it glows with colour in the polari- 

 scope, and a picture of the state of internal stress is obtained which 

 can be readily interpreted. 



It is useful to form an idea of the behaviour of a beam of 

 polarized light passing through a plate of stressed material, and 

 although modern research has shown that the light is a complex 

 electro-magnetic phenomenon, I will content myself with an elemen- 

 tary explanation based upon the wave theory, in which we suppose 

 that a Xicol's prism (A, Fig. 1), or a sheet of black glass, at a suit- 

 able angle reduces an ordinary beam of light to a transverse vibration 

 B of the ether in a plane perpendicular to the path of the ray, which 

 later passes without material change through imstressed material C, 

 but is broken up into two waves D, E, vibrating at right angles and 

 in the directions of principal stresses if the material is loaded in any 

 way. Both waves are retarded somewhat by the transparent material, 

 but one more so than the other, and if a second Xicol's F prism 



