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BELL SYSTEM TECHNICAL JOURNAL 



would visibly affect a photographic film during a ten-minute exposure. The 

 dark line that moves to the right as the negative angular rotation increases 

 results from the reflection of progressively longer wave-lengths from the 

 quartz of the main plate. The three series of exposures in Fig. 6 show the 

 progressive removal of the disturbed quartz by etching with 48% hy- 

 drofluoric acid. After ten seconds' etching the line from the disturbed 

 material does not show distinctly beyond the 1° 30' position; after 20 seconds' 

 etching it is distinct only through the 1° 00' position and after 40 seconds it 

 can only be seen distinctly at the 30' position. If the film had been ex- 

 posed for a longer time at each position the line from the disturbed material 

 at each angle would have persisted with longer etching. With the arbitrarily 



Fig. 7. — Photograph produced 1)\ rcilcciion m a broad X-ray beam from the (100) 

 cleavage face of a rock sail cr\stal (Berg) 



chosen ten-minute exposures a line from material misoriented b}' 45' dis- 

 appears after about 25 seconds' etching with 48% hydrofluoric acid, but the 

 disappearance of the 30' line occurs only after 70 seconds' etching, which 

 removes an amount of quartz equivalent in weight to a layer about four- 

 tenths of a micron thick on each surface. 



With this technique we are measuring the more grossly misoriented sur- 

 face material, material that is probably not continuous with the quartz of 

 the main plate. This is evident from the fact that a piece of quartz would 

 have to have a length-thickness ratio of 26 to 1 to take a 3° deflection without 

 breaking and the microscopic evidence does not indicate the presence of any 

 such long, thin pieces of quartz attached to the plate. 



