of the double refraction in strained glass. 



479 



The experiments described below were undertaken with the 

 intention of testing the exactness of this law throughout the 

 whole of the visible spectrum, Wertheim's result being derived 

 from only three kinds of light, viz. sodium, neutral tint, and red 

 glass viewed by transmission, which latter is hardly homogeneous. 



2. The apparatus employed in the experiments was as follows. 



A glass beam AB (Fig. 1), about 30 cms. long, 3*65 cms. 



height and 2*9 cms. thick, was placed on two knife-edges G, D, 



EX 



Fig. 1. 



12 cms. apart. Long wooden arms PQ, RS were attached to the 

 projecting parts AG, BD of the glass beam by a sort of fish-plate 

 arrangement shown in Fig. 1. 



Suspended from these arms by vertical strings hung a long 

 wooden beam EF. When the latter was loaded in the middle with 

 a weight W, the effect was to apply forces W/2 at Q and S and 

 therefore a uniform bending moment Wa/2 throughout the part 

 of the beam between the knife-edges, a being either distance QG 

 or SD. 



Besides this moment Wa/2 which could be applied at will 

 there was always a permanent moment on the beam due to the 

 weight of the arms PQ, RS and the projecting parts of the beam 

 AB. This permanent moment, however, could be calculated, so 

 that the total moment was always known. 



The optical part of the arrangement is shown in plan in Fig. 2. 

 Two glass beams such as the one described above were placed side 

 by side. Light from a vertical slit T was first rendered parallel 

 by a collimating lens L, then passed through a 60° prism P and 

 finally focussed by a lens L' , after passing through a polarizing 

 Nicol M and through the two beams on a scale s placed on the 

 side of the front beam nearest the observer. 



