DISPERSION IN ARTIFICIAL 1MMT.I.K RKl-'i: ACTION. 



289 



Tims, the stretss-optical coefficient C (= /*, /i a for unit stress) is given by the 



approximate formula 



C=C /(l-A /X) .......... (14), 



or 



(15) 



The curve connecting and X is therefore a rectangular hyperMa. When X 

 C = oo , and as X increases without limit, C decreases to its limiting value CQ. 



6500 



6000 



5500 



5000 



4500 



4000 



25 30 



LOAD IN KILOS 



35 



O 



Best fitting line, X = 949+ 145W. 



Observations. 



Line, X = IT 



Fig. 6. Typical diagram showing relation of X to W. 



Thus, if the law continued to hold accurately for small wave-lengths, then for light 

 of the critical wave-length X,, the stress-optical effect would become actually infinite. 



No ildiilit this law is purely empirical so far, and will very probably not hold for 

 very small wave-lengths. It is, however, sufficient to indicate that, as we approach 



2 o 2 



