166 THE AMERICAN MONTHLY [June, 



true angle of the optic axes. If, however, the Adams 

 spherical cell is employed, there is no necessity even 

 here to use the special objective, as the older wide angle 

 form serves admirably. With the parallel sided cells it 

 is preferable to use the special objective. The Adams 

 sphere is not supported similarly to the rectangular cells, 

 but is conveniently held by its cylindrical neck in a small 

 support directly attached to the lower quadrant of the 

 adjusting apparatus. The measurement of the true an- 

 gle of the optic axes is then carried out in the usual 

 manner, similarly to the determination of the apparent 

 angle in air, while the crystal is immersed in the liquid 

 contained in one or other of the two forms of cell. 

 Monochromatic light should of course be used in mak- 

 ing the observations, a sodium flame some little distance 

 in front of the polarizer being employed by Prof. Klein. 

 The great advantage of this method of determining 

 the true inner angle between the optic axes lies in the 

 fact that it is totally unnecessary to prepare section 

 plates of the crystal, the whole crystal itself being em- 

 ployed, and thus material saved. Prof. Klein does not 

 claim for it the highest attainable accuracy, and for the 

 class of work such as that with which some are identi- 

 fied, the determination of the crystallographic charac- 

 ters of series of isomorphous compounds closely resemb- 

 ling each other, where every endeavor must be made to 

 attain the upper limits of experimental accuracy, such a 

 method is of course inadequate. But for the ordinary 

 description of minerals and the crystals of isolated chem- 

 ical preparations unlikely to be injured by the immer- 

 sion liquid, and particularly, for laboratory teaching, 

 the method is one of the simplest and most interesting 

 yet described. The accuracy depends entirely upon the 

 closeness of the approximation of the refractive index of 

 the liquid to the beta index of the crystal. Of course it 



