376 BELL SYSTEM TECHNICAL JOURNAL 



In order to reduce fogging due to stray light a large shield is placed 

 between the film and the light source, having a vertical slit just large 



Fig. 9 — Device on polar oscillograph to limit recording time. 



enough to allow the vibrating beam of light from the mirror to pass 

 through to the film. This device reduces the fogging due to stray 



