CRYSTALS UNDER THE MICROSCOPE 179 



of the crystals being sufficiently easily orientated to yield fairly 

 accurate measurements. 



Or, we may employ the glass hemisphere (see page 117), or 

 the orientating apparatus of Klein (page 117). 



Microscopes having fixed stages require the employment of a 

 goniometer eyepiece, consisting essentially of a cross-hair system 

 rotating in conjunction with a graduated circle. With this 

 device the centered crystal remains in a fixed position and the 

 ocular cross-hairs are rotated in such a manner that one of them 

 is first made parallel to one boundary edge, and then to the other 

 edge of the angle sought. 



Extinction Angles. 1 The extinction angle of a crystal may 

 be defined as " the angle between an axis or direction of elasticity 

 and some known crystallographic direction." The crystallo- 

 graphic direction usually adopted by chemists, where the extinc- 

 tion angle is employed as one of a series of identity tests, is the 

 longest edge of the crystal or in the case of 

 rhomb-shaped crystals the line bisecting the 

 acute angles. 



In the case of crystals exhibiting parallel ex- 

 tinction the extinction angle may be consid- 

 ered as being o degrees. Crystals exhibiting 

 oblique extinction, i.e., those of the monoclinic 

 and triclinic systems yield two extinction 



angles; but it is customary to record as the 



. . , FIG. 105. Extinction 



extinction angle the smallest angle obtained Angles e e 



between the length of the crystal (cleavage 

 lines or edges being used), and the nearest axis of elasticity. 

 In Fig. 105 the extinction angles may be considered as the 

 angles 6. 



If the analyst is sufficiently well-trained in crystallography to 

 be able to locate the c-axis he may record as the extinction angle 

 the angle formed between the c-axis and the nearest axis of 

 elasticity. This value is that most often taken by crystallog- 

 raphers as the characteristic extinction angle. 



1 See Wright, Measurement of Extinction Angles; Am. J. Sci. (4), 26 (1908), 

 349- 



