

Doubly Refracting Plates and Elliptic Analyzers 27 



ing plate whose order is approximately % , mounted edge to edge 

 with an angle of 90 between their principal azimuths. It may 

 be considered as composed of two halves whose principal azi- 

 muths are the same and whose orders are equal but of opposite 

 sign. 



The Brace Elliptic Halfshade consists of a thin piece of mica 

 covering only half the field, and mounted in balsam between care- 

 fully selected cover glasses, free from double refraction. 



The Laurent Saccharimeter Halfshade consists of a doubly 

 refracting plate whose order is l / 2 for some selected wave-length 

 (usually the D line of sodium) and covering half the field. In 

 its best form, it is either mounted in a medium of the same aver- 

 age refractive index, or the other half of the field is covered by 

 a glass plate of the same average index. 



All three may be considered as special cases of a halfshade 

 whose two halves have the same principal azimuth, but different 

 orders, and which transmit the same amount of unpolarized light. 

 The emergent light is analyzed by a.nicol. 



If the incident light is referred to its major and minor axes, the 

 angle between the major axis and the principal azimuth of the 

 halfshade being 2£(o,l), between the halfshade and nicol ^ ( 1,2), 

 and the orders of the two halves of the halfshade, A r x and N\, 

 the following equations result : 



P=P 



M M o 



1—e 2 



Q P °i+e\ (16) 



K=o 



„ ^. 2<f 



S=P — V 



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

 Since 



P 2 =P =P Q—Q\=° 



or, from (14) 



183 



