150 MINERALOGY 



be held perpendicular to the crystal faces, as the true angle is 

 obtained only in this position ; after satisfactory adjustment the 

 angle is read from the card. This model also has the advantage of 

 giving both the actual angle between the faces and the supplement 

 to it or that between the poles, which is the angle usually recorded 

 in the description of crystals. 



With smooth faces t and as large as a centimeter across, the angles 

 between them may be measured with the contact goniometer to 

 within one degree, an accuracy sufficient for the identification of 

 forms and species. Fig. 301 is a more expensive instrument, in 

 which the two arms are detached from the scale and one moves 

 along the other, which enables one to measure crystals separated 



by reentrant angle. To 

 gain experience in the use 

 of the instrument it is well 

 for the student to measure 

 the angles and identify the 

 faces on a distorted Her- 

 kimer County quartz crys- 

 tal of about 1.5 cm. in 

 diameter. 



When more accurate 



work is required, as in 

 FIG. 301. Contact Goniometer. , , .. . ., 



the calculations of the 



crystalline constants or characters of any mineral and in the 

 determination of the indices of the faces as well as the identifica- 

 tion of new forms, the reflecting goniometer is used. 



There are several varieties of this instrument. One, the single- 

 circle goniometer, in which the angle is measured between the poles 

 of the two faces in question. Another, the two-circle goniometer, 

 by which the pole of any face is located, in reference to some chosen 

 face as the base. The face may be said to be located by its lati- 

 tude being measured on one circle and its longitude being meas- 

 ured on the second circle at 90 to the first, just as a point on the 

 earth's surface is fixed. There is also a more complicated instru- 

 ment in which three graduated circles are used. Of these instru- 

 ments only the single-circle goniometer will be described. 



The card contact goniometer may be easily converted into a single- 

 circle reflecting goniometer, for use in measuring crystals too small 

 to yield results sufficiently accurate by the contact method of 

 measurement. If a bridge be cemented on the arm over the eye- 



