MICROSCOPIC TECHNIQUE AND REA(JEXTS. 775 



ing as to whether C or a is the axis bisecting the acute angle, the 

 ACUTE BISECTRIX, Bxa, the Crystal is called optically positive 

 or optically negative. Thus if Bx,, = C, the optical character is 

 POSITIVE. The apparent angle between the optic axes is deter- 

 mined by means of an eye-piece micrometer in an observation of 

 the interference figure, looking along the acute bisectrix of the 

 optic axes, and this angle is designated as 2E. The character 

 of the double refraction may be determined by this angle." 



It is not possible in this work even to attempt to treat of the 

 principles underlying the study of physical crystallography. The 

 study is one requiring special laboratory instruction. ( )f the 

 excellent works which the student will find useful the followinf,^ 

 may be mentioned : 



P. Groth : Physikalische Krystallographie, 4th Ed., 1905. 



Theodor Liebisch: Grundriss der Physikalischen Krystallographie, 1896. 



Henry A. Miers: Mineralogy, 1902. In this work will be found several 



excellent chapters dealing with the principles of the measurement of 



crystals and the study of their optical properties. 

 RoSENBUSCH AND WuLFiNG I Mikroskopische Physiographie der Mineralen 



und Gesteine. 

 P. Groth : An Introduction to Chemical Crystallography. Translated by 



Hugh Marshall, 1906. 



In the Zeitschrift fi'ir Krystallographie will be found refer- 

 ences to the crystallographic studies which have been made upon 

 some of the important plant constituents, but as these studies 

 were mostly made upon relatively large crystals, which could be 

 measured and examined by means of the goniometer, these 

 observations must be interpreted and applied to crystals which 

 are formed upon microscopic slides. 



A rather large number of substances have been examined and 

 only a few of the more important are included at this time. 

 While drawings might have been made to illustrate the form of 

 crystals and optical orientations, it was deemed advisable to use 

 some of the photo-micrographs made by the author. The four- 

 color plate (Figs. 99, 100) is introduced to show the chromatic 

 eflfects observed by using crossed nicols. The plate illustrates 

 salicin and cocaine hydrochloride and is a nearly exact reproduc- 

 tion of the eflfects obtained with the micro-polariscopc. the electros 



