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364 THE ADVANCE OF PHOTOGRAPHY
course has been synchronized with the transmitting cylinder.
It is possible to obtain more detail by this method than by Korn's, for the lines are somewhat closer together than in that process. The time of transmission is, however, considerably greater, being about twentytwo minutes.
In this method too, a certain amount of current always flows through the circuit. Belin used batteries of forty to fifty cells, but the process can be worked quite satisfactorily by a very much smaller number.
A third method, which is due to Mr Thorne Baker of the Daily Mirror, is one which, owing to its simplicity and dispatch, is likely to come to the front. It is known by the name of the Telectograph process, and has been in use since July 1909, for transmitting pictures from Manchester to London, and also from Paris to London.
To better understand the principle which underlies this method, let us consider the following simple experiment.
Soak a piece of white blotting-paper with a solution of potassium iodide in Avater. Place wires which are connected to the terminals of an electric cell on this wet paper, so that the points of the wires are a short distance apart. If the distance is not too great a current will pass through the wires, as the wet paper is a conductor if a poor one. When this takes place a brown mark will appear where the positive wire touches the paper. This brown mark is really produced b}^ a tiny deposit of iodine, which has been set free by the action of the electric current upon the potassium iodide present. A method based upon this simple experiment has been in use for a considerable time for telegraphic writing.
If now Ave turn to the account given on p. 266 of the half-tone process, we shall be able to understand how it is possible for a picture to be composed of separate dots or lines of \Tarying densities and yet not appear harsh or in any degree Avanting in gradation. Noaa% if it can be