Almost everybody is familiar with the terms “positive” and “negative” as used in photography. The finished paper print is a positive, with light and shade in the correct positions; while the glass plate from which the print is made is a negative, with light and shade reversed. The lantern slide also is a positive, and it is exactly like the paper print, except that it has a base of glass instead of paper, so that it is transparent. Similarly, a positive may be made on a piece of celluloid, and this, besides being transparent, is flexible. The first step in transmitting on the Korn system is to make from the photograph to be telegraphed a positive of this kind, both transparent and flexible. This is bent round a glass drum or cylinder, and fixed so that it cannot possibly move. The cylinder is given a twofold movement. It is rotated by means of an electric motor, and at the same time it is made to travel slowly along in the direction of its length. In fact its movement is very similar to that of a screw, which turns round and moves forward at the same time. A powerful beam of light is concentrated upon the positive. This beam remains stationary, but owing to the dual movement of the cylinder it passes over every part of the positive, following a spiral path. Exactly the same effect would be produced by keeping the cylinder still and moving the beam spirally round it, but this arrangement would be more difficult to manipulate. The forward movement of the cylinder is extremely small, so that the spiral is as fine as it is possible to get it without having adjacent lines actually touching. The light passes through the positive into the cylinder, and is reflected towards a selenium cell; and as the positive has an almost infinite number of gradations of tone, or degrees of light and shade, the amount of light reaching the cell varies constantly all the time. The selenium therefore alters its resistance, and allows a constantly varying current to pass through it, and so to the transmission line. At the receiving end is another cylinder having the same rotating and forward movement, and round this is fixed a sensitive photographic film. This film is protected by a screen having a small opening, and no light can reach it except through this aperture. The incoming current is made to control a beam of light focused to fall upon the screen aperture, the amount of light varying according to the amount of current. In this way the beam of light, like the one at the transmitting end, traces a spiral from end to end of the film, and on developing the film a reproduction of the original photograph is obtained. The telegraphed photograph is thus made up of an enormous number of lines side by side, but these are so close to one another that they are scarcely noticed, and the effect is something like that of a rather coarse-grained ordinary photograph.