n8 



PHOTO-TELEGRAPHY 



high relief in the photograph happens to be at the 

 point S the stylus is pressed outwards an amount d, 

 so that the small platinum wheel W fixed at the end 

 of this arm is displaced an amount d X M, if the 

 distance from F to R be M times as great as that 

 from S to F. 



The movement to and fro of this little wheel is 

 therefore always in accordance with the relief of 



FIG. 55. 



the photograph. It travels over a very small 

 rheostat, consisting of several coils of different 

 resistances, one end of each coil being soldered to 

 a thin copper plate, the other end to one unit of 

 the telephone line. These copper strips arc each 

 separated by a narrow strip of insulating material, 

 and the top surface worked dead flat, so that the 

 wheel W travels backwards and forwards over 

 them ; the surface of this rheostat should in reality 



