140 



BELL SYSTEM TECHNICAL JOURNAL 



aging.) Although a plate lapped with 180 carborundum can be easily dis- 

 tinguished from one lapped with 303^ emery by a comparison of intensity 

 ratios, plates lapped with nearly similar abrasives cannot be distinguished 

 with certainty, except on a statistical basis. 



That the disturbed material measured by this technique differs in orienta- 

 tion from that of the main plate by less than a few minutes is shown by the 

 fact that the range of incident angles over which ionization-detectable X-ray 

 reflection takes place from a lapped crystal plate is the same within the 



REVOLUTIONS OF PLATE ON LAP 

 15 20 25 30 35 40 45 



50 55 60 



30 40 50 60 70 80 SO 

 ETCHING TIME IN SECONDS 



Fig. 3. — Effect on reflection-intensity produced by lapping on etched plate with 303^ 

 emery and soap solution and by etching the lapped plate with 48% hydrofluoric acid at 

 25°C. 



limits of error of measurement as that for reflection from an etched crystal 

 plate. 



2.3 Photographic Measurement of Angular Misoreentatiox with 

 THE Single Crystal Spectrometer 



A technique which does indicate quartz misoriented by more than a few 

 minutes has been devised by Dr. C. J. Davisson. Although the X-rays 

 reflected from this material are too weak to produce a measurable current in 

 the ionization chamber, they will darken a photographic plate or film if ade- 

 quate exposure time is allowed. The principles of this technique are illus- 

 trated in Fig. 4 and some of the resulting photographs are shown in Figs. 

 5 and 6. 



The plate to be measured is placed at the Bragg angle to the incident X- 

 rays as determined by preliminary measurement of the maximum ionization 



