Doubly Refracting Plates and Elliptic Analyzers 29 



a match with plane polarized light (t=o) only, independent of 

 the value of ^(0,1), and thereby greatly simplifies the 

 formulae. The Bravais biplate when properly made is a bal- 

 anced halfshade, but those made commercially do not always 

 show perfect balance. This quality would make the Bravais bi- 

 plate preferable to the Brace halfshade, were it not for the 

 greatly increased sensibility obtainable with the latter (see dis- 

 cussion of sensibility, section 10). Experiments are now in progress 

 with a view to producing a balanced Brace halfshade. 



Laurent Sacchariuieter Halfshade. — If as a special case e=o, 

 then t=o and the light is plane polarized. In case tg27riV^o, a 

 match is obtained only when sin 2(o,i)=o. This is the condition 

 of the Laurent Saccharimeter halfshade. It is evidently useful 

 only when it is known that the solution investigated introduces 

 no ellipticity, unless the plate should be an achromatic half-wave 

 plate. 



9. Elliptic Halfshade Systems With Compensator 



The halfshade may be either a Bravais biplate or a Brace ellip- 

 tic halfshade. The compensator may be any plane parallel doubly- 

 refracting plate, preferably one whose order, N<J4, because of 

 the greater chromatic dispersion of higher orders. For use with 

 the Brace elliptic halfshade, the Brace compensator is preferable 

 because its chromatic dispersion is proportional to that of the 

 halfshade. The Brace compensator is constructed like the half- 

 shade, but the piece of mica covers the whole field instead of only 

 one-half. 



Two arrangements will be considered : 



A. ^^-^ Elliptic Halfshade s-»- Compensator s->- Nicol. 



B. s- >- Compensator s-> Elliptic Halfshade s-*- Nicol. 



For a match of intensity of the two halves of the field : 



But since in general, P d =P'-i^P„, Q3=QV 



185 



