COMMERCIAL photo-telegraphy may be said to have started in November, 1907, when Prof. Arthur,Korn installed three of his selenium instruments at the offices of the Lokal Anzeiger, in Berlin, L'Illustration, in Paris, and the Daily Mirror, in London; towards the end of 1908 a further selenium apparatus was installed at Manchester. These early machines depend on the sensitiveness to light of one modification of the element selenium—selenium of the slate-grey form distributed over two platinum coils wound one between the other over a flat rectangular plate of steatite, being termed a selenium cell. Current was passed from a battery through the cell, on which was cast illumination from a Nernst lamp, the rays of which had first to pass through a revolving transparent photographic film, so that the intensity varied each instant according to the density of the photograph. A second cell was illuminated simultaneously by suitable means, the two being connected up on opposite sides of a Wheatstone bridge, so that the current sent to the receiving machine varied as the difference of the reciprocals of the resistance of the two cells. By combining suitable cells the inertia was largely overcome, and a photographic portrait could be transmitted in twelve minutes. The current at the receiving station passed into a string galvanometer, and laterally displaced a small shutter attached to the “strings,” this movement cutting off more or less of the light projected from a second Nernst lamp on to a sensitive photographic film revolving synchronously with the transmitting cylinder, and one-quarter its size.