480 Notes on some New Polarizing Prisms, 



each, making the ends of the prism A E and F C reversed in 

 position, but inclined at 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 " re- 

 versed " Nicol, in which the crystallographic axis lies very 

 nearly in the plane of the end-face, and in which the balsam- 

 film is very nearly at right angles to the crystallographic 

 axis. Or, comparing the two : — 



Reversed 

 Ordinary Shortened 



Nicol. Nicol. 



Obliquity of end-face ... 71° 69° 



Angle between end-face and 



crystallographic axis ... 45 5 



Angle between balsam-film and 



crystallographic axis ... 45 94 



The result is that the blue-iris limit is thrown right back, 

 and a shorter prism is obtained, having an equally wide field 

 or wider. 



Fig. 7 shows the same method applied to a slightly longer 

 piece of spar, producing a " reversed " prism of precisely the 

 same external form as the ordinary Nicol, and having indeed 

 everything the same save the direction of the crystallographic 

 axis, as a comparison of figs. 5 and 7 will show. 



The method of " reversing " the section is of course equally 

 applicable to flat-ended Nicols. If a piece of spar is first cut 

 so that the terminators are orthogonal to the long edges of 

 the prism, it is obviously just as easy to slice the prism with 

 a section that is very nearly perpendicular to the crystallo- 

 graphic axis as to slice it with one that makes only 45° 

 with it. 



This new method of construction may be regarded as a 

 compromise, for the sake of cheapness, between the method 

 of Hartnack and the older method of Nicol. 



The prisms exhibited by the author at the British Associa- 

 tion Meeting at Aberdeen consisted of a " reversed " and of 

 a " reversed-shortened " prism; both of them cut for the 

 author by Messrs. Steeg and Keuter, of Homburg. 



