120 PHOTO-TELEGRAPHY 



tiveness is not of great consequence, because the 

 change in intensity of the current is not very rapid 

 nor periodic as it is in the case of the transmis- 

 sion of a single-line half-tone photograph. But it 

 renders the obtaining of very fine details uncer- 

 tain, one merging into the other or being lost 

 altogether. It is of course possible to increase the 

 sensitiveness of the galvanometer by using a more 

 powerful magnetic field, and by using longer wires, 

 but where oil is used in the latter case there would 

 be more friction owing to increased field of sur- 

 face tension, and I think that eventually M. Belin 

 will be obliged to utilise the methods of damping 

 the vibrations that have been applied to the string 

 galvanometer in place of oil. 



Now let us see how the photographic image is 

 formed. A Nernst lamp N (Fig. 55) is placed 

 at such a position that a pencil of light con- 

 centrated from it upon the mirror M is reflected 

 upon a diaphragm G. This diaphragm has a 

 rectangular aperture, so that the light reflected 

 from M on to it when M is in the zero position 

 falls upon one end of the aperture ; as the mirror 

 swings to one side, so the light falls more towards 

 the other end of the aperture, etc. Now this 

 aperture, which had much better have been a 

 triangular one as Korn uses in his selenium 

 machines, is covered with a graduated sheet of 

 glass, which Belin terms his " scale of tints." It 



