Till: MEASUREMENT "I CRYSTALS 



L68 



FIG. 304. 



divided to half degrees and with the vernier may he read to min- 

 This circle is revolved by the pilot wheel f, which in turn 

 may !>e damped to the axis of the instrument by the set-screw b. 

 This circle may he accurately turned to any part icular point by fii-t 

 revolving; it with the pilot wheel and then setting the screw a and 

 u<in- the fine adjustment, or tan- 

 gent screw F. 



The crystal to be measured is 

 cemented with wax to a carrier 

 which fits in a socket in the table 

 of the instrument and is held in 

 place by the screw p; the screw d 

 allows this table to he elevated or 

 lowered until the edge to be meas- 

 ured may he seen in the telescope. 

 In order to adjust the crystal 

 there are four movements neces- 

 sary, each of which is controlled 

 by a separate screw: two, r and q, are screws at right angles 

 to each other and allow the carrier to be pushed back and for- 

 ward; and the two screws n and o are connected with sections of 

 cylinders, the axes of which are at 90 ; these allow the crystal 

 to be tilted in planes at 90. By these four screws the edge to 

 be measured may be quickly brought to coincide with the axis 

 of the goniometer and therefore at right angles to the plane of 

 reflection, and the edge after adjustment will also be in focus when 

 the lens D is before the telescope. 



Measurement of a crystal. Let the crystal selected for measure- 

 ment be one of topaz from Thomas Mountains, Utah, as these 

 crystals are combinations of forms of several zones, and usually 

 they possess bright smooth faces. The crystal is first cleansed with 

 alcohol and ether and then not touched with the fingers to dull the 

 faces and spoil their reflecting qualities. The various zones pres- 

 ent are noted and rough sketches or sections at right angles to 

 each zone made; each face in the zone sketched is represented 

 by a letter on the drawing and the letter placed opposite the 

 reading taken of the face when the angles are measured; this 

 enables each reading of the goniometer to be referred to the 

 right face. 



The crystal is now mounted on a carrier with the edges of the 

 /one to be first measured perpendicular to its surface; the carrier 



