52 PHOTO-TELEGRAPHY 



More contrast could be got in the received pic- 

 tures by placing over the diaphragm a graduated 

 glass, made by exposing a photographic plate to a 

 fixed light some distance away and gradually 

 uncovering it, so that on development one end 

 is very dense, the other very transparent, density 

 graduating into transparency. By cementing such 

 a glass to the diaphragm so that at the apex you 

 have a dense part gradually becoming more trans- 

 parent as you approach the base, the effect is to get 

 far more light through as the shutter uncovers the 

 triangular hole than would be the case otherwise. 



About the time of the first trials with Korn's sele- 

 nium apparatus, the submarine telephone lines link- 

 ing this country with France were rather faulty, 

 and it was only occasionally that we were able to 

 have line for experimental purposes. The average 

 business telephone call lasts six minutes, and in 

 these early days we wanted it for at least half an 

 hour ; hence, when one or more lines were " down," 

 the pressure on the others was so great that the 

 telephone authorities were quite unable to give us 

 the use of one. The consequence was that experi- 

 ments were few and far between, and often we found 

 the line quite good for the purposes of ordinary 

 telephony unsuitable for our own trials. This 

 state of affairs continued for some months, but in 

 the spring matters improved, and we were able to 

 make numerous experiments, generally obtaining 



