28 



PHOTO-TELEGRAPHY 



there is considerable lag, and it may actually take 

 some seconds before the resistance becomes the 

 normal for no illumination. 



Now let us see what this means in actual photo- 

 telegraphy. It has been seen that the transparent 

 photograph revolves in cylindrical form, so that 

 different consecutive parts of it intercept the light 

 beam which, after reflexion in the prism, falls on 

 the cell. Suppose a bright part in the photograph 



Y 



O I ( ( icrmn u.f t 071 



FIG. 9. 



is adjacent to a very dark part, the light falling on 

 the cell is great at the moment the bright part inter- 

 cepts the light, and is very small immediately after- 

 wards when the dark part takes its place. The 

 current passed through the cell should increase with 

 the bright part and instantly fall again when the 

 dark part comes ; instead, the lag in the cell inter- 

 venes, and it only half falls, and thus interferes 

 fundamentally in the process. 



Various means have been tried to counteract the 



