PHOTOGRAPHS BY TELEGRAPHY 337 



the selenium machines. The wire crosses the optic axis, light 

 from the Nernst lamp l being intercepted by the wire, so that 

 the shadow of the latter covers the slit s. When current 

 flows, the shadow shifts, and light passes through the slit and 

 is concentrated on a point of the revolving sensitive film 

 attached to the receiving drum d, which is revolving syn- 

 chronously with that of the transmitter. 



Prof. Korn also utilises a weak current flowing through 

 the galvanometer in the opposite direction to that of the 

 received current, so that an actual break of current never 

 takes place. A condenser k is placed across the transmitting 

 units to prevent sparking at the style. 



Here it is interesting to note that, working between Berlin 

 and Paris, when 60 volts is used over telephone lines, about 

 15 milliamperes of current usually enters the receiving apparatus. 

 Using an earth " return," instead of having a metallic circuit 

 throughout, as much as 5 milliamperes has been received. The 

 rate of transmission is ten minutes for pictures 12 x 12 centi- 

 metres in size. M. Chatenet, who operates the apparatus in 

 Paris for the Daily Mirror, has done much experimental work 

 with me in preparing half-tone photographs on metal-foil for 

 transmission, but despite the very low moment of inertia of the 

 moving part of Korn's galvanometer, an oscillation is set up 

 owing to the regular period of the lines in the half-tone image, 

 and the results so far have not been altogether satisfactory. 

 M. Chatenet and myself have both come to the conclusion that 

 single-line half-tone photographs are the best for transmission, 

 and such results are solely used for transmission in my own 

 telectograph. 



Half-tone photographs may be prepared in a variety of ways. 

 The most suitable for electrical transmission are those made 

 with a single-line screen, by copying the original in a camera 

 fitted with a glass screen ruled with thirty-five or forty parallel 

 lines to the inch. This screen is placed a short distance in 

 front of the plate, and the latter on development renders the 

 subject as a series of lines varying everywhere in width 

 according to the densities in the photograph. A print is made 

 from the half-tone negative so obtained upon a sheet of metal- 

 foil coated with fish-glue, sensitised to light by means of 

 potassium-dichromate. The print after exposure is washed 

 in water, when the unexposed parts (the "whites") dissolve 



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