460 



SUMMARY OF CURKENT RESEARCHES RELATING TO 



6 = 1- 336. It crystallizes in similar form to ealcite, and in both 

 cases thin plates obtained by cleavage may be used. 



As the cementing substance for the nitrate of soda, a mixture of gum 

 dammar with monobromonaphthalene was used, which afforded an 

 index of refraction of 1 * 58. In the case of thin plates of ealcite, a 

 solid cementing substance of sufficiently high refractive power was not 

 available, and a fluid medium was therefore employed. For this pur- 

 pose the whole prism was inclosed in a short glass tube with air-tight 

 ends, which was filled with monobromonaphthalene. In an experi- 

 mental prism a mixture of balsam of tolu was made use of, giving a 

 cement with an index of refraction of 1'62, but the low refractive 

 power* resulted in very considerable reduction of the field. The extent 

 and disposition of the field may be varied by altering the inclination at 

 which the crystal lamina is inserted (fig. 72), and thereby reducing the 

 length of the prism, as in the case of the Hartnack. 



Fig. 72. 



Fig. 73. 



In order to obviate the effects of reflection from the internal side 

 surfaces of the prism, the wedge-shaped blocks of glass of which it 

 is built up may be made much broader than would otherwise be 

 necessary; the edges of this extra width are cut obliquely, and 

 suitably blackened. 



The accompanying diagram (fig. 73) represents a prism of cylin- 

 drical external form constructed in this manner, the lower surface 

 being that of the incident light. In this the field amounts to 30°, and 

 the breadth is about double the length. 



Dr. Feussner remarks that a prism similar in some respects to 

 his new arrangement was devised in 1869 by M. Jamin,t who used a 

 thin plate of calc-spar inclosed in a cell filled with bisulphide of 



* i. e. low as against 1 • 6585 the greater index of the calc-spar. 

 t Comptes Kendus, Ixviii. (1869) p. 221. 



