X-RAYS AND ORIENTATION OF QUARTZ CRYSTALS 307 



ray by the angle 29. If, when the crystal gives a reflected ray, it is rotated 

 in its own plane on the reference points the reflection is unchanged. 



(b) Atomic plane intersecting plate-face in a line normal to the plane 

 of the instrument.^ 



If in the above case the rotation of the crystal in its own plane does 

 cause a change of ionization current the surface is not parallel to the atomic 

 plane. Figure 3.11 illustrates a case in which the surface is at an angle 5 

 to an atomic plane, this intersection lying normal to the plane of the paper 

 in Fig. 3.11. In order that the angle between the X-ray beam and the 

 atomic plane shall be 6, the angle between the beam and the plate-face 

 must now he 6 — b on one side and 6 -\- b on the other. The deviation 

 angle 29 is the sum of these two. Therefore for atomic planes intersecting 



Fig. 3.10 — Gonionietry case (a): .Atomic plane parallel to plate-face 



Fig. 3.11 — Goniometry case (b): Atomic plane intersecting plate-face in a line normal to 

 "the plane of the instrument" 



the plate-face in a hne normal to the plane of the instrument the ionization 

 chamber is set at 29 regardless of the angle between the atomic plane and 

 the surface of the plate. 



In Fig. 3.11 the entering angle is shown as ^ — 5, the leaving angle as 

 9 -[- b. If the crystal is now rotated through an angle of 180° in its own 

 plane the entering angle will have to be set at 6 -|- 5 and the leaving angle 

 will become 9 — b. The crystal holder will have been moved through an 

 angle 25. Thus b can be found by observing the angle through which the 

 crystal holder must be rotated in order to achieve X-ray reflection from 

 the same atomic plane when the crystal is rotated 180° in its own plane. 



^ The plane of the instrument is a plane normal to the axis of rotation of the instrument 

 and containing the incident ray. 



