Optical Deter mi nation of Stress. 



713 



These readings give a mean value for Young's modulus 

 or 299,100 in lbs. and inch units, and for Poisson's ratio a 

 value 0-39. 



For comparative purposes a set of similar values are given 

 in Table II. for a piece of plate-glass which when examined 

 between crossed nicols showed very little trace of internal 

 stress, and in this respect it was very similar in character to 

 xylonite. 



The specimen* was 1*016 ins. by 1*008 ins. in section and 

 1'25 ins. of measured length. 



Table II. 



Load, Compressions in 

 pounds. raillionths of an inch. 



Lateral extensions, 

 iniilionths of an inch. 



1000 

 3000 

 5000 

 7000 

 9000 

 7000 

 5000 

 3000 

 1000 



240 240 



480 24 ° 

 700 240 



940 22 ° 



o-;a 



7°0 " _U 



m «» 



2J S M0 



4°2 42 

 44 



134 « 



and the mean value of E obtained from these readings was 

 10,380,000 and for Poisson's ratio 0'233. 



If a homogeneous beam of plane-polarized light passes 

 through a plate of unstressed glass, xylonite, or other like 

 transparent material, it suffers no decomposition; but the 

 application of a tension or compression stress causes the 

 material to behave like a double refracting substance, and 

 the plane-polarized beam breaks up into plane-polarized rays 

 having their directions of vibration parallel and perpendicular 

 to the axes of principal stress. These rays have different 

 velocities in the material, and their relative retardation R is 

 proportional to the indices of refraction /j, q and //, e of the two 

 rays and the thickness T of the plate of material through 

 which they pass; this is expressed by 



R=Oo-/*)T 



(1) 



Experiments on glass show that the difference of the re- 

 fractive indices is proportional to the difference of the principal 



* Specimen " d," p. 65, of "An Investigation into the Elastic Constants 

 of Rocks," by Adams and Coker, Proceedings of the Carnegie Institution, 

 Washington. 



