JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. 9 DECEMBER 4, 1919 No. 20 



PHYSICS. — The relation between birefringence and stress in 

 various types of glass. L. H. Adams and E. D. WiIvIvIam- 

 SON, Geophysical Laboratory, Carnegie Institution of Wash- 

 ington. 



When a rigid body is subjected to stress there are produced 

 changes in refractive index which are related to the direction of 

 stress and to the vibration directions of the entering light. Iso- 

 tropic substances such as glass then become birefracting. It 

 is evident that birefringence may serve as a measure of strain 

 and, consequently, of stress; indeed this method is commonly 

 used for the determination of internal stress in glass, and is of 

 especial importance in the examination of optical glass, which 

 for use in lenses and prisms must be well annealed. 



While it is common practice to speak of strain in glass in 

 terms of birefringence, little is known concerning the absolute 

 magnitude of the stresses or strains involved,^ and, as a part 

 of the general problem of glass annealing, definite information 

 concerning the relation of birefringence to stress was required. 

 Accordingly in this paper we present the results of some measure- 

 ments of the birefringence due to loading of nine kinds of op- 

 tical glass. A brief review of the optical effect of stress is also 

 given. 



EXPERIMENTAIv METHOD AND RESULTS 



Blocks of each kind of glass measuring about 2 by 3 by 3 cm. 

 were prepared. The faces were made as nearly plane parallel 



1 The only observations known to us are those by Pockels with reference to several 

 glasses, some of which were flint glasses, while the others were unusual alumino- 

 borates. See page 615 and 620. 



609 



