1895.] MICEOSCOPICAL JOURNAL. 169 



crystal, a parallel section-plate were being examined, 

 the natural faces of the crystal — however, rich in faces 

 the zone may be — not entering into consideration what- 

 ever. 



The advantages of the use of an immersion liquid of 

 equal refractive power in the examination of crystals 

 have been pointed out by several previous observers. 

 In 1841, Biot, in his memoirs concerning lamellar polar- 

 ization, describes the use he made of it. The method 

 has long remained dormant, however, as far as is known 

 from the literature of this branch of study. In the 8th 

 Ed. of the Lehr. der Physic unci Meter, it is stated that 

 if the refractive index of the liquid in which a plate per- 

 pendicular to one of tlie medium lines is immersed is 

 equal to that of the crystal, the true angle between the 

 optic axes is at once afforded. Latterly, however, the 

 evident advantages of the method have suggested them- 

 selves to several crystallographers. M. Fougue mentions 

 it in his memoir in the Bulletin of the French Miueralog- 

 ical Society of 1894 on the felspars. 



The writer of this article has frequently made use of 

 the method for certain specific purposes and it may be 

 of use to other workers to give a brief indication of one' 

 or two modes of extending its sphere of usefulness not 

 touched upon by Prof. Klein. In the course of the in- 

 vestigation of the normal sulphates of potassium, ru- 

 bidium, and caesium, (Jour. Chem. Soc. 1894, 628) a 

 difficulty was found in determining the true optical ax- 

 ial angle of rubidium sulphate by means of the very 

 accurately orientated section-plates prepared by use of 

 the new grinding goniometer described to the Royal So- 

 ciety, {Phil. Tra7is. 1894, A 887) earlier in the same 

 year. The difficulty, which is one not uncommonly met 

 with, was owing to the fact that the extremely low 

 double refraction, necessitating the use of very thick sec- 



