Till, Ml. AM Kl.MKNT ()l- CRYSTALS 



Measurement of the pyramid zone. "['lie crystal is now removed 

 from the holder and remounted with one of the edges o! tin- pyra- 



iniii /one perpendicular to the ll;it surface of tin- holder. It is 

 then dumped in the goniometer and an edge adjusted as before; 

 the edge first selected for measurement should be that between the 

 unit pri-m and the first pyramid, as all the pyramids are in the zone 

 \vitli the unit prism. The first reading is taken from m and con- 

 tinued around the /one, taking the faces in order through the 180 

 until m" is reached. The following are the results of measure- 

 ments in this pyramid zone: 



It is seen that the face c is 90 from the prism and is therefore 

 the base; its parameters and indices are oo &: oo b: c, (001). 



The second pyramid zone, including the 

 prism faces m'", m", may be measured and c 

 averaged with the readings of the first. 

 From the above results the indices of tin- 

 pyramids present are determined graphi- 

 cally as follows. Some one of the pyramids 

 must be chosen as the unit pyramid or the 

 fundamental form in order to arrive at the 

 unit on the axis c; for this pyramid u has 

 been chosen. To determine the length of 

 c. draw, Fig. 307, oc the vertical axis and 

 om at right angles to it, making om = om 

 of Fig. 306, as om is the trace of the zonal 

 plane, at right angles to the prism face. 

 It is the plane in which all the poles of the 

 pyramids lie and is therefore the plain- in which all the angles have 

 been measured; then in Fig. 307, from m draw mi, making 



Fio. 307. 



m 



