2S2 O. Palache — Crystal Measurement, etc. 



the face is centered. H is now brought to the reading h^ and 

 the reflection from / centered on the crosshairs by use of the 

 adjusting tables. When properly adjusted, that is when paral- 

 lel to Y, the reflected signal should remain stationary on the 

 crosshairs during a revolution of Y. 



Case II. The pole face {/) defined by a zone perpendicular 

 to it (prism zone in general). 



An edge of the prism zone is centered and adjusted to the 

 horizontal crosshair. H is clamped at ^^ + 90° and the reflec- 

 tions from two adjacent faces of the zone are brought to the 

 center of the crosshairs by means of adjusting tables. H 

 is then loosened,, the edge again centered and the whole 

 repeated as often as necessary. Other faces of the zone are 

 then examined and if necessary the adjustment bettered. When 

 the prism zone is thus adjusted at ^^+90, /", the pole face, is at 

 h^ and if present its reflection should be on the crosshairs 

 when ^is brought to h^. This case is important since the 

 plane at right angles to the prism zone is the normal plane of 

 projection in all systems and this position gives simple and 

 direct results in the calculation. The prism zone is rarely 

 lacking entirely and even when its faces are poor or striated its 

 correct adjustment is possible. 



For adjustment in the many other cases that may arise ref- 

 erence is made to the original papers.^ The crystal once 

 adjusted, each face is brought to reflection by movements of 

 H and Y and the angles (/> and p observed. If both ends 

 of the crystal are developed, it is inverted after measurement 

 of one end, the same zone being made equatorial and the 

 same meridian being used. If the readings in the new posi- 

 tion be 0' p^, then 



180° -p' = p. 



The value </)„= h^ is subtracted from all the values found for 

 <f>. The value p^ = v^ (for the first meridian) is similarly 

 subtracted from all the values for p and the resulting values, 

 (j) — ^^ = (p^,p—p^ — p^ are the angular coordinates of the faces. 



The angles thus determined for each face are evidently quite 

 independent of any other face and give an absolute picture of 

 the crystal as it is, quite free from the approximations and 

 averages introduced when the measurement is by zones as is 

 now usual. If desired they give likewise the interfacial angles 

 and (what cannot be measured by the other method) the angles 

 of the zones or edges. 



*V. Goldschmidfc, G-oniometer mit zwei Kreisen, Zeitschr. fur Kryst., xii, 

 3 893, p. 215. Polarstellen am z weikre isigen Goniometer, idem, xxiv, 1895, pp. 

 610-613. 



