li)lo] 



on Great Advance in Crystallography 



(;<s<.) 



<ind left-handed (iiuirtz crystals are so well known that it will 

 not be necessary to do more than illustrate the fact by c.iie or 

 two experiments. First of all, two slides are shown (repro- 

 duced together in Fig. 12), one of a right-handed crystal and the 



FiC4. 10. — Barlow's Con'ception 

 FOR Right-handed Quartz. 



Fig. 11. — Barlow's Conception 

 FOR Left-handed Quartz. 



Other of a left-handed one, each exhibiting the characteristic 

 little faces s and x of the right and left trigonal Iji-pyramids and 

 right and left trigonal trapezohedra, modifying the right and left 

 corners formed by the meeting of the faces of the rhombo- 



Fig. 12. — The Two Varieties op Quartz Crystals, 

 Left-handed and Right-handed. 



hedron r and hexagonal prism m. That the right-handed crystal 

 shows rotation of the plane of polarization of light to the right, 

 and the left-handed crystal optical rotation to the left, may be 

 beautifully demonstrated by cutting a plate 1 mm. thick out of each 

 crystal perpendicularly to the axis, and using the device of 24 mica 



