188 BELL SYSTEM TECHNICAL JOURNAL 



Instead of using the constants tth and ttu it is customary to use a single 

 constant C given by 



B = ijie - fJio = r = CT (103) 



where the constant C is called the relative stress optical constant and r the 

 retardation. The dimensions of C are the reciprocal of a stress and are 



Fig. 3 — Photoelastic picture of a disk in compression. 



measured in cm- per dyne. A convenient unit for most purposes is one of 

 10~^'* cmVdyne; if this is used, the stress optical coefficients of most glasses 

 are from 1 to 10 and most plastics are from 10 to 100. This unit so defined 

 has been called the "Brewster". In terms of the Brewster, the retardation is 



r = CTd (104) 



If C is measured in Brewstcrs, (/ in millimeters and 7' in l)ars (10^ dynes/ 

 cm'-) then r, as given by the formula, is expressed in angstrom units, 



