The advance of photography : its history and modern applications (1911)

PHOTO-TELEGRAPHY Everyone is more or less familiar with the fact that it is now possible to take a photograph in Manchester or Paris, and transmit a copy of that photograph by telegraphy to London. Such pictures were first of all sent to the Daily Mirror in the latter part of 1907. Two or three methods of accomplishing this task have been in use since that time. The first method which came into practical use was one devised by Prof. Korn of Munich. His system depends for its success upon the variation in the electrical resistance of a selenium cell when acted upon by a more or less powerful light. Suppose the photographs to have been taken upon a film, then certain parts of that film, when developed, will more easily transmit light than others. Xow let this film be placed round a transparent c}7linder, and a source of light so arranged that by means of a lens or system of lenses the rays are focussed on the film. Then more or less light will pass through the film and transparent cylinder to be reflected by the rightangled prism upon the selenium cell, according as to whether a less dense or more dense part of the photograph is in the path of the light (see fig. 161). The resistance of the selenium cell varies according to the degree of illumination, and so the current passing through the circuit in which the cell is will also vary, as the amount of light received by the cell changes. The axis on which the cylinder is placed rotates and at the same time gradually rises, so that the whole length of the film comes in time in the path of the rays. The next step is to have at the receiving station some