4 PHOTO-TELEGRAPHY 



mark on the paper. Hence, when the style has 

 traced over the entire length of the sketch at A, the 

 latter will be reproduced (negatively) at B. By 

 suitable means it can, of course, be received posi- 

 tively if desired. 



Such is the system which is now over sixty years 

 old, and many trials were made with it to telegraph 

 writing over distances ; here, however, the diffi- 

 culties met with in long cables were at once felt ; 

 attempts were actually made at one time in France 

 to use such a system commercially, but they were 

 soon abandoned. 



Thence onwards continuous attempts were made 

 to solve the problem of transmitting sketches, 

 pictures, and photographs ; a long list of names of 

 these early workers might be given. But we may 

 well confine our attention at present to two men, 

 Amstutz and Shelford Bidwell, as the ideas of these 

 men were actually the germs of two important pro- 

 cesses which have now given most satisfactory 

 results. The latter made use of the newly dis- 

 covered sensitiveness to light of the metal selenium, 

 the former of the possibility to use the relief in a 

 certain form of photographic image to vary the 

 strength of the electric current. 



On the lines followed by Bidwell, Ayrton and 

 Perry also made experiments, both therefore utilis- 

 ing selenium at the sending station. Let us suppose 

 that a portrait is to-be telegraphed from this station 



