590 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



15° from the X axis, since it was obtained by setting the analyzer and 

 polarizer at 15° from the direction X. Counting the number of isochro- 

 matic Hues from the "eye", the principal stress difference T{ — T2 is 

 known and using equation (30), T^ is at once calculated. To obtain 

 dT^/dy, it is sufficient to divide Tq by the distance y that the point is 

 above the X axis. Proceeding this way, values of dT^/dy are plotted as 

 a function of X. Then from the first of equations (32) with Ti^ = at 

 the edge, one can obtain Ti by integrating dT^/dy dx over the X axis. 

 The result is the curve marked Ti of Fig. 5. Having Ti , T2 can be ob- 

 tained from the isochromatic lines which determine T2 — Ti and this 

 is plotted on the upper part of Fig. 5. 



