WRIGHT: MEASUREMENT OF STRAIN IN GLASS 595 



series of ingenious experiments many of the fundamental laws 

 of the optical behavior of glass strained either mechanically by 

 differential pressure or tension, or as a result of non-uniform 

 heating or cooling. Brewster found: that a plate of glass under 

 load is birefracting; that the optical effect produced is sensibly 

 proportional to the intensity of the strain; that a plate of glass 

 under differential compression behaves optically as a uniaxial 

 negative crystal with its principal axis in the direction of the act- 

 ing load, while under differential tension it acts as an optically 

 positive uniaxial crystal; that in a glass plate cooled quickly 

 from a high temperature a permanent strain is imparted which 

 is at maximum intensity next to the outer surfaces (zone of 

 compression) , and which, decreasing toward the center, reaches a 

 neutral band and passes then into a zone of tension in the central 

 part of the plate; that compression produces retardation, while 

 dilatation causes acceleration of the transmitted light waves, 

 with the result that a rapidly cooled cylindrical rod of glass with 

 plane parallel end surfaces behaves as a negative lens (meniscus) . 

 Following Brewster, Fresnel- proved definitely that two waves 

 are transmitted through a strained glass plate; F. E. Neumann^ 

 gave a mathematical solution of the general problem on the basis 

 of certain assumptions; A. Wertheim'' verified Brewster's state- 

 ment of the proportionality between optical effect and intensity of 

 strain; Mace de Lepinay^ found that the path difference is practi- 

 cally the same for all wave lengths and that, therefore, the 

 interference colors are essentially those of Newton's color scale. 

 J. Kerr*' proved, by means of a Jamin interference-refractor, that 

 in the case of compression both waves are retarded while in the 

 case of tension both waves are accelerated; that the wave whose 

 vibrations take place in the plane normal to the direction of the 

 applied load is retarded most, its retardation being practically 

 twice that of the wave vibrating along the direction of strain; 

 that "the action of direationally strained glass, in the common 



- Oeuvres Comi)letes. 



3 Pogg. Ann. 54. 1841. 



* Comptcs Rendus, 32, 33, 34. 1854. 



f* Ann. d. Chim. et Phys., 19: 1-90. 1880. 



« J. Kerr, Phil. Mag., (5), 36: 321. 1886. 



