MICRO-ANALYSIS. 821 



right angles to each other, and these are designated as fl, the 



axis of greatest elasticity; U, the axis of mean elasticity; and 

 C, the axis of least elasticity.* 



"(5) The position and angle of inclination of the optic axes 

 or lines of single refraction through the crystals. These always 



lie in the plane of the elasticity axes fl and C and the angles 

 between the optic axes are bisected by the axes Q. and C Accord- 

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

 ACUTE BISECTRIX, Bxa, the Crystal is called optically positive 

 or optically negative. Thus if Bxa=^t, the optical character is 

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

 termined by means of an eye-piece micrometer in an observa- 

 tion 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. Of the 

 excellent works which the student will find useful the following 

 may be mentioned : 



P. Groth : Physikalische Krystallographie, 4tli 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 : Mikroskopische Physiographie der Min- 



eralien 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 ob- 



* Elasticity in the optical sense is the reciprocal of refractive index; 

 hence a, 6, C, are the axes of least, mean and greatest refractive index. 



