105G SUMMARY OF CURRENT RESEARCHES RELATING TO 



Hartnack prism) are totally reflected out, because the crystallographic 

 axis slopes at 45°. 



To remedy this tlic crystal is cut in the manner shown in fig. 226 

 or fig. 227. Fig. 226 represents a piece of spar of the same size as 

 fig. 225. The end-faces are first cut away about 40° each, making the 

 ends of the prism A E and FC reversed in position, but inclined at 



Fig. 226. 



about 69° instead of 71° to the long edges. The prism is then cut 

 across E F, which makes about 89° with the end-faces. The result 

 is a shortened and " reversed " nicol, in which the crystallograpliic 



Fig. 227. 







Br; 



b 



axis lies very nearly in the plane of the end-face, and in which the 

 halsam-film is very nearly at right angles to the crystallographic axis. 

 Or, comparing the two, — 



Ordinary Reversed 



Nicol. shortened 



Nicol. 



o o 



Obliquity of end-face 71 69 



Angle between end-face and crystallographic 



axis 45 5 



Angle between balsam-film and crystallo- 

 graphic axis 45 94 



