With the apparatus of Prof. Korn, of Munich, described in these columns, only photographs of faces can be transmitted to a distance, since pictures of busts and landscapes give poor results owing to insufficient distinctness. This problem, however, has been solved in a much more perfect manner by a Frenchman, M. Edouard Blin, as has been shown by the experiments performed by him in the laboratory of the Socit Franaise de Photographie. The transmitting apparatus of this ingenious inventor is wholly mechanical in all its details. A carbon print of the photograph to be telegraphed is placed on a revolving cylinder, while a stylus traveling over this print imparts to the line conductor by means of a lever current differences corresponding with the differences of relief through a rheostat. In transmitting, the picture is rolled on a horizontal cylinder of metal. The picture consists of a carbon print made on rather thick paper, and presenting a relief proportional to the intensity of the colors of the picture. This difference of level, almost unnoticeable to the touch, is, however, sufficient for a point in gliding over the cylinder to respond to the differences and to transmit them in movements of a corresponding amplitude to the extremity of the arm of a lever. When the cylinder is turned, the fine sapphire point of the stylus passes over the entire surface of the picture, following spiral lines one-sixth of a millimeter apart. The differences of relief of the picture are translated into movements at the other end of the lever, and by means of a little slider on a rheostat connected with the line, currents of an intensity proportional to the amplitude of these movements and therefore the corresponding colors are sent. In receiving, the apparatus follows the same general principles as that of Prof. Korn. As already described in these columns, Korn has made a receiver in which a beam of light is projected on the sensitized surface upon which the impression is to be made through a galvanometer in which a little sheet of aluminium forming a shutter is displaced more or less according to the intensity of the currents transmitted by the line, communicating to the luminous ray correspond- ing intensities. In the receiving apparatus of Blin we find the same source of light produced by a Nernst lamp, but the galvanometer is supplanted by an apparatus known as the Blondell oscillograph, from the name of the inventor. This oscillograph is made up of two large coils between which oscillates a very small mirror scarcely two or three millimeters in diameter, and of an extreme sensibility of movement. The movements of the mirror, like those of the aluminium sheet of Korn, are strictly proportional to the intensity of the received current. The ray coming from the Nernst lamp reflected by this oscillating mirror is projected on a lens, where it produces a luminous line which is displaced more or less from right to left, according to the oscillations of the mirror. Opposite this lens, a sheet of glass is placed, called the "color scale," tinted gradually from right to left, from black to absolute transparence. According to the zone where it is projected, the luminous ray is colored more or less, while the lens on whatever point of which it falls focuses it back to the fixed point where it impresses itself upon the photographic paper. This paper is rolled upon a cylinder contained in a dark chamber and is displaced before the point of light with a rotary motion, identical, with the exception of the correction for synchronism, to that of the sending station. To avoid spreading of the luminous ray so as to produce a halation around each point, which would cause a lack of distinctness in the picture, the cylinder almost touches the wall of the dark chamber in which is pierced the extremely small hole through which the luminous ray, reduced to a surface of one-sixth of a millimeter, enters. The inventor can, at will, obtain a print sent as a negative or as a positive, whether it comes from a transmitting positive or a transmitting negative. It is merely necessary to transpose the color scale so that the ray of light may be tinted in a manner directly or inversely proportional to the intensity of the current and that of the colors sent. It is equally possible to receive a print of a desired tone, whether it comes from a picture either normal, too feeble, or too intense, by employing color scales of a proper transparence. Finally, enlargements can be made by replacing the receiving cylinder with another of larger diameter. In the demonstrating apparatus here shown, a transmitting and a receiving station are mounted on the same table and worked by the same electric motor. Hence, they need not be adjusted for synchronism, as the inventor intends to do later. The imaginary line which unites the two stations is represented by a series of coils, consisting of a resistance equivalent to a line of about 750 miles in length.
This article was originally published with the title "The Belin Process of Telephotography" in Scientific American 97, 25, 456-457 (December 1907)