312 BELL SYSTEM TECHNICAL JOURNAL 



WW axis using the atomic plane (01 -1). (In actual practice the closer 

 plane (02-3) is used.) Here: 



5 = 49° - 38°13' - 10°47', d = 13°20', 26 = 26°40' 



From which ^ = 4°57', g = 13°35', g + g' = 26°34' 



Thus if the plate is correctly cut the reflected ray will enter an ionization 

 chamber whose slit is long enough to receive rays which make an angle of 5° 

 with the plane of the instrument. However, if the plate were in error by 

 more than 6' around the XX axis (Fig. 3.13) jS would excede 5° and the 

 center of the reflected beam would not enter the ionization chamber. In 

 actual practice, therefore, the length of the ionization chamber slit should 

 be enough greater than that required for the calculated /3 to admit reflections 

 from erroneously cut plates. 



Since the diff"erence between g + g' and 26 is only 6' and since the width 

 of the ionization chamber slit is usually great enough to accept reflected 

 beams over a range of several times 6', no correction of the ionization 

 chamber position may be necessary in this case. 



On the other hand, the orientation of the plate with respect to the inci- 

 dent beam is highly critical and since g differs from <? by 15' this correction 

 in the orientation of the quartz plate must be made. 



Discussion of the general case (d), in which the intersection of the atomic 

 plane and the plate face is neither normal nor parallel to the plane of the 

 instrument, will be found in Section 3.9. 



3.7 Choice of an Atomic Plane for Checking the Orientation of 



Any Given Face 



If the plate-face to be checked does not lie parallel to an atomic plane, 

 the nearest usable atomic plane must be found and the orientation of this 

 atomic plane in the plate must be determined. The procedure for these 

 two steps is outlined in this and the following sections. 



The problem of the choice of an atomic plane for checking the orientation 

 of one of the faces of a given plate has two parts: 



(A) Determination of the orientation of that face with respect to the 

 A", Y and Z axes of the mother crystal and (B) discovery of the atomic 

 plane whose orientation and X-ray properties are most suitable for use 

 with that face. 



(A) Determination of the orientation of the face with respect to the 

 A', Y and Z axes of the mother crystal. 



The orientation of the plate is commonly given in terms of the Ai , .I2 

 and A3 angular rotations as described and illustrated in Section 2.4 of 

 Chapter II. Briefly, a basal section of crystal is placed initially with its Z 

 axis vertical and its +A' axis toward the operator of a horizontal axis saw, 



