PRACTICAL EXPERIMENTS 121 



is merely a photographic negative having practi- 

 cally no silver deposition at one end of the rect- 

 angle, and graduating to a dense deposit at the 

 other end. As the mirror swings to one side, 

 so the light from it falls on a denser portion 

 of the scale of tints, and less light emerges from 

 the glass. 



Behind the diaphragm is a condenser C, which 

 refracts the collected light and brings it to a point 

 on the drum D%, round which a sensitive film is 

 wrapped. The intensity of this spot of light is 

 proportional to the current, hence inversely to the 

 relief in the photograph. The drum D 2 travels 

 sideways as it revolves, the pitch of thread used 

 being the same as that in the transmitter. It 

 travels in a light-tight box fitted with a tube and 

 diaphragm, as in the case of the telautograph. But 

 M. Belin has happily arranged that the one 

 cylinder serves both as transmitter and receiver. 



In 1907 and 1908 Belin made various experi- 

 ments over a long-distance telephone line, with the 

 two apparatus in one room under observation. 

 Thus he had two lines between Paris and Lyons 

 linked up at Lyons, the two machines being at Paris, 

 and the current had therefore to travel to Lyons and 

 back while passing from the transmitter to the 

 receiver. 



His method of synchronising differs little from 

 that already described, but in the most recent 



