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BELL SYSTEM TECHNICAL JOURNAL 



atomic planes are at an angle 5 to the axis of the instrument, the reflected 

 ray will not lie in the plane of the instrument. If, however, the vertical 

 angle jS, between the reflected ray and the plane of the instrument is small, 

 the reflected ray may still enter the ionization chamber. The maximum 

 angle /3 which is permissible, is not the same for all instruments, but de- 

 pends on the vertical length of the slit. In most cases it is about 5°. 



The proper angular settings of the instrument are no longer 6 for the slab 

 and 26 for the ionization chamber but are the orthogonal projection of 

 these angles onto the plane of the instrument; g and g + g' , respectively. 



AXIS OF ^[ 



INSTRUMENT 



ATOMIC PLANE 



PLANE OF 

 INSTRUMENT 



PLATE-FACE 

 NORMAL 



Fig. 3.14 — Goniometry case (c): Atomic plane intersecting plate-face in a line parallel 

 to "the plane of the instrument" 



Formulae are given below for j8, g, and g -\- g' in terms of the Bragg 

 angle 6 and the angle between the atomic planes and the surface of the 

 quartz plate, 5. 



sin /3 = 2 sin 6 sin 5 (3.3) 



cos {g -f g') = 



sm g = 



cos 26 

 cos (8 



gin 6 

 cos 5 



(3.4) 



(3.5) 



The relations between /3, g + g', g, 6, and 5 are shown by the curves 

 in Fig. 3.15. 

 As an example, suppose that a BT plate is to be corrected about the 



