THE TRANSMISSION OF PHOTOGRAPHS 



BY TELEGRAPHY 



By T. THORNE BAKER, F.C.S., F.R.P.S. 



Many attempts were made to telegraph pictures some twenty 

 years ago, but the methods then conceived were not developed, 

 in all probability on account of the lack of possible application. 

 Modern journalism is such, however, that a continual and liberal 

 supply of what are called "news photographs" is demanded, 

 and the time has thus arrived when photo-telegraphy can hold a 

 useful position and perform a commercial function. 



There are two ways in which a photograph can be made to 

 control or actuate the electrical currents necessary to transmit 

 it over a distance to a suitable receiver; one is optical, the other 

 mechanical. We shall consider each of these in turn. 



Optical Method of Transmission 



In the optical method we must depend on the density of the 

 photographic image to control the amount of light falling upon 

 some body whose electrical properties vary with intensity of 

 illumination. Selenium, in the crystalline form, is extremely 

 sensitive to light, its resistance being reduced by approximately 

 fifty per cent, when illuminated by the maximum amount of light 

 suitable for influencing it. Its sensitiveness is greatest towards 

 the orange region of the spectrum, though Ruhmer claims to 

 have sensitised it for special regions by the aid of optical 

 sensitisers such as are used in orthochromatic photography. 

 The sensitiveness to different regions of the spectrum, according 

 to measurements made by me in 1907, are as follows : 



