16 PHOTO-TELEGRAPHY 



this arc, the poles of which are represented in the 

 diagram by PP, is placed at the focus of a parabolic 

 reflector RI, and its rays are therefore reflected as 

 a parallel beam to the receiving reflector R 2 . At 

 the focus of this second reflector is a selenium cell 

 Z, whose resistance is altered by the light falling 

 upon it from the reflector. This cell is in series 

 with a battery and mirror galvanometer, light from 

 a lamp N being reflected by the mirror on to a 

 graduated aperture H ; the collected light is 

 focussed upon a photographic film attached to the 

 drum 2, which revolves synchronously with the 

 transmitting cylinder Ci. 



The idea is an ingenious one, and might be made 

 to work in practice over distances of a few hundred 

 yards, but not more. A suggestion was made on 

 somewhat similar lines to these to the author by 

 Mr. Sharman, in reference to Korn's selenium 

 machines, but for the purposes of wireless tele- 

 graphy the fluctuations in the resistance of the 

 selenium would be used to influence the undamped 

 oscillations given out by a singing arc, and a suit- 

 able receiver would record these fluctuations 

 photographically . 



One other possible means of receiving from any 

 form of transmitter over short distances deserves 

 reference, inasmuch as it has recently received the 

 attention of Rignoux and Fournier for their pro- 

 posed television apparatus. It is well known that 



