Doubly Refracting Plates and Elliptic Analyzers 



33 



compensator. A 7- ,. For an error not greater than i per cent, 

 to be introduced by using the approximate formula ( 57) , 2irJV 2 

 must not be greater than 11. 5 in any case, and the further a lies 

 from 45 , the smaller the allowable value. Since the angle be- 



Ma-Tf Sha.de Adjustment (a.) 



Fi g- 3 



tween halfshade and nicol regulates the sensibility of the system 

 and is necessarily small for large values of A A", the approximate 

 formula can only be used where AA' is small enough to give sat- 

 isfactory sensibility at 45 °. For large values of AA 7 , as in the 

 Bravais biplate (&N=%), the approximate formula is therefore 

 not available, and it would be necessary, if this arrangement were 

 used, to fall back upon the rigorous formula (55). As this 

 involves the necessity of reading not only the compensator but 

 also the halfshade rotation, it is preferable to use the arrange- 

 ment B. 



If the halfshade is not balanced, a conveniently usable formula 

 can only be obtained when ^(1,3)— 45 , i.e. when the halfshade 

 is placed at 45 ° to the analyzing nicol. This of course requires a 

 sufficiently low value of A A 7 " to give satisfactory sensibility. Letting 



2£(o,i) = — $, and 2^(2, 3) =1/', then 2^(1,2)= i// and the formula 



(54) reduces to 



189 



