Eaki.e.] 



Col em unite from Southern California. 



41 



Polar Orientation . — Every face of a crystal has its position 

 denned with reference to a pole and a direction assumed as first 

 meridian, when its two angular distances, respectively, from 

 these are known. By means of the graduated vertical and 

 horizontal circles of the instrument, these two angular coordi- 

 nates, 4> and p, are readily derived. The plane, normal to the 

 prismatic zone is preferably chosen as the pole-face, and the 

 great circle passing through the pole and the normal to the side 

 pinacoid (010) as the first meridian. In systems with rectilinear 

 axes the pole-face would then correspond to the basal-pinacoid, 

 and the angles for the three pinacoids would be 



<j> P 

 001 0°00 0°00 



oio o oo 90 oo 



100 90 oo 90 oo 



Since monoclinic crystals have no plane normal to the pris- 

 matic zone, the position of such a plane is best defined if 

 prismatic faces are present. Colemanite possesses a well-devel- 

 oped prismatic zone, including both pinacoids, so the polar 

 orientation of the crystals was readily accomplished. Each face 

 of a crystal comes into reflection when it is normal to the line 

 bisecting the angle between the telescope and collimator of the 

 instrument, and this normal position must first be determined. 

 Its angle on the horizontal circle H, is the Ji reading for all 

 measurements. The method of finding //,, is quite simple. 

 Having the telescope tightly clamped at a convenient distance 

 from the collimator, a blight reflecting surface is then mounted 

 approximately parallel to the vertical circle, I', and centered. 

 It is then brought at the intersection of the crosshairs by turning 

 H and V, and the reading on H taken, — Jt\. V is then turned 

 180° and the reflection again brought into position by the 

 adjustment tables and H. This reading on H — ~h%. Then is 

 h = Hhi — h%) < This can be repeated until the .reflection 

 remains rigidly fixed at the intersection of the crosshairs, during 

 a revolution of 1". This final position of H is then the 7? , and 

 need never be changed. 



The crystal of colemanite was mounted with its prismatic 

 zone approximately normal to 1", and H was clamped at 



