286 C. Palache — Crystal Measurement, etc. 



by a tightly-fitting ball-and-socket joint, adjustable by a key 

 which fits in two holes bored in the stem at right angles ta 

 each other. The crystal is affixed to the support as nearly a& 

 may be in the desired position. The prism zone, if present, is 

 preferably placed perpendicular to H or the basal plane (pole 

 face) parallel to H. If the basal plane is present, the carrier 

 with S is placed at 0° and the plate p brought nearly in con- 

 tact with the crystal face. The crystal is then tipped gently 

 by means of the key and ball-and-socket joint first in one direc- 

 tion and then at 90° to the first, until the face is parallel with 

 ^, that' is normal to S. 



If the prism zone is to be used in orienting the crystal, the 

 rod is set at 90° and two of the prism faces successively brought 

 parallel to p. In other cases the procedure is similar to that 

 for the reflecting goniometer,"^ Y and H in the two instru- 

 ments being always interchanged. The crystal being oriented, 

 the measurement is effected by bringing each face parallel to^ 

 by movements of H and of the carrier on Y. Readings are 

 taken on each circle, that on H giving 0^ = </>— </>o where 0^ is 

 the reading for the first meridian and that on Y giving 

 directly the polar elevation p^. The measurements are discussed 

 in the same manner as those on the reflecting instrument. 

 Headings are exact to within J°, depending on the size and 

 character of the faces and the practice of the observer. 



The instrument is also useful in demonstrating the principle 

 of the method, its relation to the geographical and astronom- 

 ical methods of measurements, and especially the relations 

 existing between crystal faces, their normals and their projec- 

 tions either in steorographic or gnomonic projection. f 



A further extension and application of this method of crys- 

 tal measurement is found in the latest product of Prof. Gold- 

 schmidt's inventive genius — the projection goniometer. The 

 purpose of this instrument is to construct a gnomonic projec- 

 tion directly from the crystal without either measurement of 

 angles or calculation being necessary. Angular measurements 

 may, however, be taken at the same time, and the instrument 

 thus in a measure supplies the place of the reflecting goni- 

 ometer. 



Fig. 6 shows the instrument as first constructed. The hori- 

 zontal circle H turning about the axis A^ has a plane surface 

 of brass or glass. A paper is fastened upon it to receive the 

 points of projection. H is graduated with readings to 1^ 



Resting freely on H is the carrier S, upon which is placed 

 the crystal. S consists of a heavy base K ; of a guide 11^ which 

 rests against two pegs in H and serves to keep the carrier in a 



* Polarstellen am zweikreis. G-oniometer, Zeitschr. fiir Kryst, 1895, xxiv, p. 610^ 

 f For further details concerning the use of this instrument see Anlegegoniome- 

 ter mit zwei Kreisen, V. Goldschmidt, Zeitschr. fiir Kryst, xxv, 1895, p. 321. 



