ADAMS AND WIIvI/IAMSON : BIREFRINGENCE AND STRESS 



6ll 



to standard methods.^ The optical system for measuring the 

 birefringence consisted of a petrographic microscope supported 

 at the proper angle on one post of the testing machine and in 

 such a position that the block of glass G was between the nicol 

 A'^ (Fig. i) and the low-power lens L. A graduated quartz 

 wedge, W, was in the focal plane of an eyepiece, E, on which was 

 mounted a cap nicol, N-2.. 



Upon illuminating the system with light from an ordinary 

 incandescent lamp, S, the wedge W- — which was placed with its 



Fig. 2. 



Diagram to show arrangement for determining the uniformity of loading. The 

 block of glass G is illuminated by light from S which passes through the diffusing 

 screen D and is reflected from the sheet of glass R. The thrust is applied to the glass 

 in a direction at 45 ° to the plane of polarization of the light, and the interference 

 colors produced by loading are observed through the analyzing nicol N. 



longest dimension in a horizontal position — was observed 

 through the eyepiece and nicol. When the analyzer was placed 

 so that the nicols were crossed, a series of colored lines extended 

 across the field in a vertical direction, and the position of the 

 central black Une with respect to the graduations on the wedge 

 determined the birefringence of the specimen. One division on 

 our wedge represented lo mm retardation and no difficulty was 

 experienced in reading to V2 division or 5 /xju, which was about i 

 percent of the maximum observed birefringence. The load 



' For example, see F. E- Wright, The methods of petrographic-microscopic research. 

 Carnegie Inst. Wash., Publ. 158, Chap. III. 191 1. 



