the intermittent current builds up a reproduction in black-and-white of the original photograph. In this process also the cylinders have to be “synchronized,” or adjusted to run at the same speed. Both this process and the foregoing one have been used successfully for the transmission of press photographs, notably by the _Daily Mirror_. Professor Korn has carried out some interesting and fairly successful experiments in wireless transmission of photographs, but as yet the wireless results are considerably inferior to those obtained with a line conductor. For transmitting black-and-white pictures, line drawings, or autographs by wireless, a combination of the two methods just mentioned is employed; the second method being used for sending, and the first or selenium method for receiving. For true half-tone pictures the selenium method is used at each end. CHAPTER XX WIRELESS TELEGRAPHY AND TELEPHONY--PRINCIPLES AND APPARATUS Wireless telegraphy is probably the most remarkable and at the same time the most interesting of all the varied applications of electricity. The exceptional popular interest in wireless communication, as compared with most of the other daily tasks which electricity is called upon to perform, is easy to understand. The average man does not realize that although we are able to make electricity come and go at our bidding, we have little certain knowledge of its nature. He is so accustomed to hearing of the electric current, and of the work it is made to do, that he sees little to marvel at so long as there is a connecting wire. Electricity is produced by batteries or by a dynamo, sent along a wire, and made to drive the necessary machinery; apparently it is all quite simple. But take away the connecting wire, and the case is different. In wireless telegraphy electricity is produced as usual, but instantly it passes out into the unknown, and, as far as our senses can tell, it is lost for ever. Yet at some distant point, hundreds or even thousands of miles away, the electrical influence reappears, emerging from the unknown with its burden of words and sentences. There is something uncanny about this, something suggesting telepathy and the occult, and herein lies the fascination of wireless telegraphy.