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gularity of the indentation is in proportion to the density of the sounds. To the advantage of a faithful reproduction of sound, the photo-eletric system adds the advantage of a perfect and constant synchronisation, since, even in case the film, should break, a disynchronisation would not occur, but the sound would only stop for a moment and would restart together with the film, because both are impressed onto the same support. Against these advantages, there is however, the difficulty of handling and using these films. A difference in the development, an imperfection in the emulsion, some specks of dust, a scratch on the sound picture, are sufficient to produce alterations in the sound, the consequence being a higher cost in the production, and a shorter life of the film.
In the photo-electric system both by density of light and by variable surface the reproduction of sound takes place in the same manner. A ray of light, passing through the sound picture, falls on a photo-electric cell which produces variations of current.
In the system of variable density, the light that falls on the photoelectric cells is regulated by the density of the sound pictures ; in the system of variable surface, the quantity of light that passes through it is given by the difference of the transparent surface. The variations of current passing through an amplifier composed of thermojonic valves reach the microphone of a loud speaker which in its turn gives out the sound.
Also in the photo-electric system, the reproduction of sound still shows some imperfections. The photo-electric cell, containing selenium or potassium, shows a certain amount of ertia which causes alterations although less serious than those of the disc system such as to make the talking film still far from absolutely perfect.
In the field of sound amplifiers, the various systems in use, today, are not yet capable of registering all the vibrations, and these often become superposed thus producing distortions of the sound causing unpleasant effects, especially in the reproduction of the voice.
The systems of amplification more commonly used are divided into two categories : the trumpet and the cone. The trumpet systems are less sensitive and require large dimensions if an exact reproduction of the sound is needed ; the diffusing cone system is more sensitive to the vibrations and renders the production of sound with much greater precision.
These are the systems used today by industry for the production of sound and talking films. Together with the imperfections of the means there are the difficulties of handling, which present a hard 'test for the ability of the technicians and for the purse of the industrial enterprisers.
Technical difficulties of scenes, and complex acoustic problems may be added to the already great optical difficulties. If practical experience has rendered the technique of the silent film** relatively easy, the
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