Then something has to be done to quicken the action of the current, or else the receiving drum must be made to lag behind the sending drum by the requisite amount. In some cases, too, the transmitting apparatus loses a little time in sending off the currents, and that, too, has to be allowed for, so that, all things considered, the reader will see that the successful solution of this problem is hedged about with many subtle difficulties which are probably only appreciated by those who are well acquainted by sad experience with the little vagaries of both electricity and mechanical devices. Neither of them does quite what we want it to do; each suffers from little faults, which in the case of a delicate problem like this, where a difference of a hundredth of a second would be fatal to success, introduce difficulties almost insuperable. To transmit line drawings, Professor Korn uses a sending instrument very like that of Caselli. The picture is placed, either by hand or photographically, upon a sheet of copper foil, which is fixed round the rotating cylinder, the lines being formed of non-conducting material. The foil being electrified and the stylus connected to the "line" or main wire, currents pass to the farther end just as in the old apparatus. At the receiving end the drum is covered with photographic paper and enclosed in a light-tight box. Through a hole in this box a fine pencil of light passes from a lamp, suitable lenses being used to ensure that the pencil shall have, as it were, a very fine point, producing a very small spot of light upon the paper. If the light remains quite steady, the drum meanwhile rotating, a line will be drawn by it upon the paper which will be visible when the latter is developed. Since the drum not only turns upon its axis, but also moves endwise one hundredth of an inch at every revolution, this line will be a spiral, the turns of which will be one hundredth of an inch apart. Thus the paper will be blacked, practically uniformly, all over. Should the intensity of the light vary, however, the line will at times be lighter than at others, while, should it be cut off altogether for a moment, then there will be a corresponding gap in the line, and it is easy to see that if these lighter parts or gaps occur in the correct places they will form a picture. In other words, by controlling that light we can build up a picture upon the paper. The question is how to control it. Professor Korn uses a form of the Einthoven galvanometer already described. Instead of the silvered fibre generally employed in this instrument, a silver wire is fitted, the movement of which partly or entirely cuts off the pencil of light.