C. Palache— Crystal Measurement^ etc. 



281 



position of the crystal face with reference to the arbitrary 

 coordinates. They may be termed the angular coordinates 

 (Winkelcoordinaten), of the face. If the equator is taken 

 parallel to Y and if H is the first meridian, then angle ^ 

 corresponds to the geographical longitude, angle p to the pole 

 elevation (complement of the geographical latitude). 



In practice, whenever possible, the crystal is so mounted that 

 the prism zone is perpendicular to Y ; or, what is the same, the 

 plane perpendicular to the prism zone, a cube face in regular 

 system, a pinacoid in hex., tetrag., and orthorhombic systems, 

 is taken as 1\\.q pole, A face of the prism zone, preferably a 

 pinacoid, is then taken as first meridian. 



H 



If this normal position cannot be used owing to absence or 

 imperfection of the prism zone, another plane is taken 2,^ pole 

 and preferably a face perpendicular to one of the other two 

 axial zones. The elements and symbols so obtained are readily 

 transformed to the normal form by interchange of axes. 



It is evident that for a given position of collimator and tele- 

 scope the value of p—h^ for the polar position of the vertical 

 circle will be constant and will serve for any plane adjusted 

 parallel to Y. This value, A^, is found as follows : A reflect- 

 ing surface is mounted, centered, and brought to reflection by 

 turning H and Y. Let the reading on H be A, . Y is then 

 turned through 180° and the surface again brought to reflec- 

 tion by turning H. Let this reading be li^. The required 

 value, Aq = 4(^1+^2)- This value may be checked and improved 

 by repeated measurements on other surfaces. Ji^ being known, 

 the adjustment of a crystal face (/*) in polar position is thus 

 carried out. 



Case I. The face (/") present and giving a good reflection. 



The crystal is fixed upon the adjustment table with hard 

 cement as near the desired position as may be and a corner of 



