Page:Popular Science Monthly Volume 91.djvu/71

 Popular Science Monthly

��weighs about fifteen pounds, is considerably smaller than the professional machine and is operated by a small electric motor. A cell of dry batteries carried in the operator's pocket supplies the necessary current. The

��Developing the film in the form of a coil re- duces the quantity of solution required. About three quarts of solution will de- velop 500 feet of film

��Below: Method of coiling the film over a wood core for immer- sion. A frilled cellu- loid strip is used as a separator between the layers of coiled film

����Corrugated celluloid separating strip

��camera can be loaded in day- light. No tripod is nec- essary.

The amateur photographer who has de- veloped his own films will require no ad- ditional knowledge for developing the paper motion-picture film. The film is wound around a wood core together with a celluloid strip frilled on the edges. The frill acts as a separator be- tween the layers of the film and at the same time allows the de- veloping solution free access to every part of the surface. The film is rinsed, fixed and washed in the usual manner. It is dried on a collapsible wooden drum. An important feature of Mr. Webb's process is the produc- tion of the positive film. The paper negative film is not trans- parent; therefore a positive film cannot be made by contact. Even if it were possible to do so, it would not prove practical for the amateur, because motion picture film printing by contact is necessarily done by machinery and entails the additional ex- pense of another length of film. A much simpler method has

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��been perfected in which the negative is chemically converted to a positive with remarkable results. One solution removes the silver nitrate from the negative ; another bleaches the shaded and dark portions leaving the film blank. On exposure to light, the color values are reversed, the most delicate tones and gradua- tions being retained in the color rever- sion.

By converting the negative into a posi- tive, it is evident that only one fin- ished positive film can be obtained from each negative. For quantity require- ments, a number of experiments have been made with the half tone or engraving proc- ess as a printing medium. In this process, the positive film is reproduced on a sensi- tized copper surface and etched with nitric acid to produce printing plates. In this manner an unlimited number of positive prints could be produced for circulation purposes, paper and ink being the only material required for the work. As the initial outlay for the half tone plates would be large, this method would prove practical only for quantities running into the

��At left : The screen on which the picture is thrown is in a shadow box

���At right : The projec- tion principle of the cabinet explained. The reflecting mirror is disposed at an angle of 45° under the lid

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