BIBLIOGRAPHY 53 



radiographs of the chest, sinus, mastoid, or small-body localization, or 

 in industrial work where castings and welded seams are examined for 

 blow holes or faults. 



3. Lag due to phosphorescence of the crystals after the x-radiation 

 exposure has ceased is caused by impurities in the calcium tungstate. A 

 cassette fitted with a screen possessing lag should be unloaded immedi- 

 ately after an exposure and not reloaded with photographic film until the 

 phosphorescence has entirely disappeared. A screen may be tested for 

 lag as follows : Lay a small piece of lead, a bunch of keys, or some metal 

 object on a cassette containing the screens under examination, but with 

 no film between the screens. Subject the empty cassette with super- 

 imposed opaque metal objects to a moderately long exposure. Take 

 the cassette to the photographic dark room and place a film in the cas- 

 sette. Close the screens over the film and allow it to stand for ten 

 minutes. Develop the film in the normal manner. If any appreciable 

 lag is present, an image of the opaque metal object will be visible on the 

 film. 



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