40 



University of California. 



[Vol. 3. 



the pole, and the latter form lies almost on the first meridian. 

 The pyramidal forms which are new are represented by two 

 circles, and the direction of the prism (310) and dome (801) by 

 donble-headed arrows. In this projection the prismatic forms 

 necessarily have no points of intersection, but their directions 

 are shown by the arrows. 



METHOD. 



The two-circle goniometer, with which the measurements 

 were made, has been fully described by Goldschmidt, together 

 with detailed instruction in the calculation and gnomonic 

 projection of the forms based on measurements with the instru- 

 ment,* and also in a briefer way by Palache.t 



Advantages. — This goniometer possesses manifest advantages 

 over the ordinary reflection-goniometer, not alone in the sim- 

 plicity and greater rapidity with which forms can be calculated 

 from its measurements, but also because the two angular coordi- 

 nates definitely locate the form, and when these angles have 

 been recorded for every known form on crystals, as Goldschmidt 

 lias done in his " Winkeltabellen," a new form can readily be 

 detected by comparing its angles with those recorded. It would 

 be practically impossible to record all of the interracial angles 

 between known forms, consequently, as often happens, the 

 measured interfacial angle is not given in the standard works on 

 mineralogy, and quite frequently after long trigonometrical cal- 

 culation the measured form turns out to be well known. The 

 instrument, however, can also serve for the measurement of 

 interfacial angles, and, in the opinion of the writer, is superior 

 for this kind of measurement, as it really requires less time for 

 the adjustment of the crystal. 



One adjustment of a crystal in true polar position is often 

 sufficient for the measurement, of all the forms, and in the case 

 of colemanite, less than an hour was required to measure the 

 most complex combination. 



*V. Goldschmidt. — Goniometer mit 'zwei Kreisen. Zeitschrift fur Krystallog- 

 raphie 1893. 21, 210-232. Die zweikreisige Goniometer (Modell 1896) und seine 

 Justirung. Idem 1898, 29, 333-345. 



t Charles Palache, — On Crystal Measurement by means of Angular Coordinates 

 and on the Use of the Goniometer with two Circles.. American Journal of Science, 

 1896 (4), 2, 298. 



