266 The American Naturalist. [March, 
MINERALOGY! 
NEW INSTRUMENTS. 
Goniometer with two Graduated Circles.—Goldschmidt’ has 
devised a new form of goniometer which he has called Goniometer 
mit zwei Kreisen. Besides the horizontal graduated circle there is in 
this instrument a vertical graduated circle, and it is this circle which | 
is fitted with the usual centering and adjusting support for the crystal. 
The vertical circle and its attachments are supported by an arm which 
revolves about the axis of the horizontal circle. The collimator and 
telescope are constructed as in the goniometer with horizontal circle, 
and for measurement are so placed that their axes make equal angles 
with the zero position of the revolving arm and movable axis. The 
crystal is adjusted for the prism zone and brought into the unmovable 
axis of the instrument. The pole of any face of a crystal is located in 
the same manner as a point of the earth’s surface by latitude or longi- 
tude, or a star by right ascension or declination. To determine a- 
plane by reflection the adjusted crystal is revolved about the movable 
axis (vertical circle) till the face is perpendicular tothe plane of the 
horizontal circle (y). The movable arm is then revolved about the un- 
movable axis until the face is normal to the zero position of the arm— 
makes equal angles with the axes of collimator and telescope (s)— 
when the image of the signal will appear on the cross hairs of the 
telescope. The position of a plane can also be determined by the 
angles through which it is necessary to turn it to make it appear as a 
line parallel to one of the cross hairs of the telescope. The inventor 
claims for the instrument, among other advantages over the forms in 
use, that measurements are more quickly and conveniently made, and 
that the calculation of crystallographical constants and symbols and 
the making of projections are much simpler. It is necessary to mount 
the crystals but once for the entire measurement, and pyramid planes 
require but a single adjustment. The position of a face is determined 
without reference to the perfection of its neighbors. Angle tables can 
be constructed corresponding to a definite setting of the crystal which 
allow the symbol to be obtained at once from the angles ¢ and s, and 
thus comparison of differently developed crystals can be easily made. 
1 Edited by Dr. Wm. H. Hobbs, University of Wisconsin, Madison, Wis. 
? Zeitsch. f. Kryst., xxi, pp. 210-232, 1893. 
