6l2 



ADAMS AND WILLIAMSON: BIREFRINGENCE AND STRESS 



could be read to o.i kg. or better — a higher degree of precision 

 than is necessary, since readings to within 5 kg. would corre- 

 spond in accuracy to the readings of the wedge. 



The first results obtained were irregular and generally unsatis- 

 factory. This was found to be due to the fact that the load 

 was not applied uniformly. In seeking to remedy this defect 

 we found that the problem of loading a block of glass evenly 

 up to a pressure of a few hundred kg. per sq. cm. offers unex- 

 pected experimental difficulties and requires the greatest pos- 

 sible amount of care and patience. Fortunately birefringence 

 serves not only for the quantitatiA^e measurement of stress but 

 also for the qualitative examination of the uniformity of stress. 



TABLE I 

 Chemical Composition and Optical Properties of the Various Glasses 



Kind of glass 



Ordinary Crown 



Borosilicate Crown. . . 

 Light Barium Crown , 

 Heavy Barium Crown 



Barium Flint 



Light Flint 



Medium Flint 



Heavy Flint 



Extra Heavy Flint. 



Si02 



73 

 67 

 47 

 40 

 46 



54 

 45 

 42 

 28 



To detect unevenness of loading a polarizer of large aperture is 

 required. A convenient arrangement is shown in Fig. 2. Light 

 from the lamp 5" is diffused by the translucent screen D (of 

 ground glass or tracing cloth) and polarized by reflection from 

 the sheet of plate glass R which is painted black on the back. 

 The vibration direction of the beam of light, as with- the previous 

 arrangement, is inclined 45° to the direction of pressure. The 

 interference colors are observed through the nicol prism A^, no 

 lens being required. If the block of glass G be uniformly loaded 

 it appears in the field of view to be of a certain uniform color 



