PHOTOTEGIE A process introduced by Couste, in roo4, for making transparencies, and reversed or duplicate negatives. The negative should be exposed as usual and developed with any developer except gyro till the deepest shadows are seen on the back of the film ; it is next well washed, and the following operations may then be conducted in daylight. Make up a solution of— Hydrochloric acid r oz. roc. ccs.
Barium peroxide . 25o grs. 57 g.
Water . . . To oz. I,000 ccs.
This should be mixed in a glass bottle, stand ing in cold water to keep it cool ; add the acid to the water, and then the barium in small quantities at a time, with constant stirring or shaking. The developed negative (not fixed) should be immersed in this solution, which should be kept rocked. The film begins to dissolve gradually, and as soon as this has well begun the barium solution is poured back into the bottle and the negative placed in water. The gelatine and reduced (developed) silver slowly dissolve away, and should any of it stick it can easily be rubbed off with a pad of cotton-wool, or the finger tip, or the barium solution may be again poured on. The result is an image consisting of various thicknesses of gelatine and unreduced (undeveloped) silver salt. The last named can be dissolved out in an ordinary fixing bath, and after well washing, the film should be stained in a solution of dye. Or, if desired, the nnreduced silver may be left in the film and redeveloped with any ordinary developer. For making duplicate negatives, it is first necessary to make a positive by contact and treat this in the manner described above, so as to reverse it —that is, turn the positive into a negative. The dyed gelatine relief serves as a negative or a positive as the case may be, the dense parts being dyed deeper than the clear parts where the gelatine is thin.
The transmission of a photographic image from one place to another by means of telegraphy. The idea of telegraphing pictures dates back to 186o, and even earlier, but it is only during the last few years that any successful results have been obtained commercially.
There are two systems which have been used extensively by newspapers : one is the invention of Prof. Korn, who was the first to produce a really practical system ; the other is the inven tim of T. Thorne Baker.
Professor Korn's By means of the original selenium process, the photograph to be telegraphed is printed as a transparency on celluloid, and this film is attached to a glass cylinder, c in diagram A. Light from a Nernst lamp N is made to pass, by the lens through the cylinder where the beam comes to a focus ; what light passes through the cylinder reaches a prism P, and is reflected on to a selenium cell S S. As the cylinder is rotated spirally (by means of a motor) different consecutive portions of the image intercept the light beams, and hence the intensity of the illumination falling on the cell s s varies. The cell has the peculiar property of varying in its electrical resistance, according to the strength of light falling upon it ; hence the current from an electric battery, E, which passes through the cell into the telephone lines connecting the sending apparatus with the receiver, also changes, and at the receiving apparatus, B, one gets an electric current varying in intensity each instant, according to the density of the photograph, as the image is traversed by the beam of light. The current received is passed through an exceedingly fine silver wire w sus pended between the poles of a powerful electro magnet If ; to this wire is attached a small shutter, which cuts off a beam of light passing from a lamp N through lens 1, and O. hole in the magnet poles. When a current passes through the wire w it is magnetically displaced, and light consequently passes through the magnet and arrives at a lens T, which concentrates it upon a sensitive film attached to a drum D ; this drum rotates in a light-tight box synchronously with the transmitting cylinder, c in diagram A. The wire, w w, is displaced to a distance depend ing on the current, which itself is regulated by the density of the photograph at each instant. Hence, the light acting on the sensitive film varies always according to the density of the picture being transmitted. On developing the film, a replica of the original photograph is obtained. There are, of course, various electrical complications in the process, but these need not be dealt with here.