SOLDERLESS WRAPPED CONNECTIONS — PART II 587 



and hence 



T[ - T2 = V{Ti - T^y + 4T|. (23) 



Hence, the birefringence is directly proportional to the difference be- 

 tween the principal stresses. 



The retardation R is the difference between the path length for the 

 fast and slow rays when they are transmitted through the thickness 

 of the specimen. If h is the thickness of the specimen 



h = V2t, h - R = vit (24) 



(25) 



(26) 



(27) 



or the number of fringes is related to the difference of the principal 

 stress by 



AT = ^ (T2 - T[). (28) 



A 



o 



For photoelastic bakelite for the green mercury line 5461 A, the fringe 

 constant \/C is 88 pounds per square inch/inch/fringe. Hence the 

 difference between the principal stresses is given by 



(T2 - T[) = ^ (88) pounds/sq in. (29) 



The isochromatic lines such as shown on Fig. 3 are taken with quarter- 

 wave plates in addition to the crossed polaroids. These lines give directly 

 the multiple number of times the stress is greater than the value (88//i) 

 pounds per square inch. In order to know the exact multiple, one has 

 to know the starting point or the points of zero stress difference, called 

 isotropic points. These are determined by using white light and locating 

 the gray fringes. The zero order fringe is gray because for this case, all 

 the wavelengths of light are delayed the same amount and hence no 



