152 MINERALOGY 



tact goniometer as just described above. The accuracy of the re- 

 sults obtained will depend upon the size of the source of light used ; 

 on the exact parallelism of the intersecting edge between the two 

 faces with the axis of the instrument ; and upon the plane of reflec- 

 tion, the plane in which the angle is measured being at exactly 90 

 to the edge of the crystal and the axis of the instrument. 



The instrument is constructed with all of these conditions in view 

 and is provided with devices allowing of adjustments to these ends. 

 Fig. 302 is the usual form of the Fuess single-circle goniometer one- 

 fourth natural size. The collimator Z is supported by the post A, 

 which is rigidly fixed to the frame of the instrument. The collimator 

 is provided with a lens at the inner end, and at the focus of this lens 

 at the outer end the slit admitting light is placed. The shape and 

 size of this opening may be adjusted in the more expensive instru- 

 ments to suit the work at hand. The usual form is that illustrated 

 in Fig. 303, and known as the Websky signal slit, hourglass in shape, 

 with itfe vertical plane of symmetry vv' parallel to the axis of the 

 instrument and fixed in this position. 



The telescope B is supported by a similar post but attached to 

 the disk or circle upon which the vernier of the scale is marked, and 

 can be revolved about the axis of the instrument, and rigidly fixed 

 in any required position by the set screw C. The telescope is fitted 

 with an eyepiece Q, which is provided with cross hairs at right 

 angles, one of which is fixed parallel to the axis of the instrument, 

 the other will be at 90 to the axis. The eyepiece is adjusted to 

 this position by the collar which clamps on 

 the eyepiece and which fits in a notch in the 

 drawtube of the telescope, thus always assur- 

 ing the correct position of the vertical hair 

 when the eyepiece is withdrawn and re- 

 turned to its position. 



The two cross hairs should divide the 

 Websky signal orthorhombically when the 

 telescope is set directly opposite the collima- 

 FIG. 303. The Websky tor, as illustrated in Fig. 304. The telescope 

 eyepiece is adjusted to parallel rays; and 



when it is wished to view the crystal being measured, the lens 

 D is placed before the tube and focuses the rays on the axis of 

 the instrument. This lens may be revolved out of the field, when 

 the signal will appear if a face is in position. 



The circular disk shown under d is the graduated circle, which is 



