154 MINERALOGY 



is now clamped in the instrument. Let the crystal be first mounted 

 on the flat basal cleavage and the prism zone the first zone to be 

 measured, the edges of which are all at right angles to the basal 

 cleavage. The crystal is elevated or depressed by means of the screw 

 d until it is in the plane of reflection. With the screw b set and 

 the screw a loose, the telescope is revolved until its axis is at about 

 110 with the collimator and there set with the screw C. The lens 

 D is placed before the telescope and the goniometer light before 

 the Websky slit in the collimator; the crystal is now adjusted. 

 With the right hand on the screw q and the axis of this screw at 

 90 to the telescope, and the left hand on the screw n, the crystal 

 is pushed back and forth with the right hand until an edge is seen 

 in the telescope, when with the left hand this edge is tilted until it 

 is parallel to the vertical hair, and it is placed directly on the hair 

 with the right hand. The crystal is now revolved with the pilot 

 wheel f 90 to the right and the right hand is placed on the screw 

 r and the left on the screw o ; the same edge is adjusted to coincide 

 with the vertical hair as before. After these adjustments have 

 been carried out accurately, the crystal when revolved will turn 

 on the edge as an axis. This edge will now lie in the axis of the 

 instrument and will therefore be at 90 to the plane of reflection and 

 in position to measure. 



The crystal is now rotated until one of the adjacent faces to the 

 edge adjusted is seen in the telescope to reflect the light from the 

 collimator, the lens D is lifted and the signal will appear. The sig- 

 nal is revolved by means of the pilot wheel f to the vertical hair ; 

 with the screw a set, the hair is made to exactly divide it, by means 

 of the tangent screw f ; if the adjustments have been accurate, the 

 two hairs will divide the signal as illustrated in Fig. 304. If they 

 do not, they are made to do so by slightly readjusting the crystal. 

 The crystal is now revolved until the adjacent face reflects the 

 signal ; and as this is in the same zone, it will need but little ad- 

 justment to bring the signal into symmetrical position. Before 

 taking any readings both signals should come into position with a 

 simple revolution of the crystal and without any adjustment of the 

 screws connected with the crystal ; when this is the case, the crys- 

 tal is in position to measure. 



It is always best to take the first reading, in the measurement 

 of a zone, near the zero on the circle, but on the 359 side and to 

 revolve the crystal so as to decrease the number of degrees in each 

 succeeding reading; they will then all stand in the column, so 



