X-RAYS AND ORIENTATION OF QUARTZ CRYSTALS 



315 



a point between (21-3) and (21 -2). Then, in the direction of the east- 

 west arcs, .1 unit is marked off to the right of the X axis. The point so 

 located is found to be about half way between the .7 and .8 concentric 

 circles, that is, to have a Z component of about .75, as calculated jibove. 

 It lies about half way between atomic plane normals (21-3) and (31-3). 



The degree scale given in Fig. 3.7 may be used to determine roughly the 

 angle between any two points on the diagram. The scale is non-uniform 

 and for any particular region of the diagram that portion of the scale should 

 be used that falls on that region when the zero mark on the scale is placed 

 at the center of the diagram. 



INSTRUMENT 



PLANE OF 

 INSTRUMENT 



Fig. 3.16 — Goniometry case (d): Atomic plane intersecting plate-face in a line which 

 is neither normal nor parallel to "the plane of the instrument" (general case). 



In the present example the part of the scale to be used is around the 40° 

 mark and the plate face is found to make angles of less than 10° with both 

 (2l-3) and (31-3). 



In general the atomic planes with the smaller indices are Hkely to give 

 the stronger reflections. Therefore the better plane to try first is (21 -3). 

 The indices of this plane are not shown in this form in Table I, but by the 

 method described in Section 3.5 it may be seen that a plane with indices 

 11-3 would have the same 6 value for X-ray reflection. From Table I we 

 see that 6 for atomic plane 11-3 is 32° 03'. 



In order to determine the angles g and g -\- g' (Fig. 3.16) the orientation 

 of the atomic plane with respect to the plate edges must be found. 



