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434 Transactions of S.M.P.E., Vol XI, No. 31, 1927
Laboratory Methods and Equipment
It is necessary that motion picture film be cleaned at various stages in its progress from the laboratory to the theater and also after its use. A mixture of ammonia, water, and alcohol is satisfactory for cleaning the base side of the negative or positive film. To remove dust and finger markings from negatives before printing, wiping with silk plush moistened with pure carbon tetrachloride is recommended. The flexibility may be restored by passing it through a bath containing a mixture of water and alcohol. ^^
A pneumatic film squeegee has been developed for use in the laboratory to remove excess moisture after washing, before drying. A 25% saving of time is effected through the use of the air squeegee and subsequent polishing of the film is unnecessary.''^
A method of impregnating wood with paraffin has greatly increased the value of wooden tanks for photographic solutions. Spruce so treated was found to withstand the action of acid and alkaline solutions with a minimum of absorption and consequent swelling. '^^
Trioxymethylene in the presence of sodium sulphite can be used to replace the alkali in the preparation of various phenolic developers. A fixing bath is recommended for photographic papers which employs trioxymethylene instead of alum as the hardening constituent. This is more stable than an acid-alum bath.'^^
Lenses
An anastigmat lens said to be three times as fast as the //2.7 has been placed on the market. It is claimed to give improved perspective, the finest delineation and modeling, to be free from focal differences with the various stops, and to have complete correction for all colors of the spectrum. '^^
A patent has been granted on a projection lens having a short back focus, permitting the lens to be placed close to the gate. The focal length of the front component is equal to the sum of the focal length of the whole lens plus twice the back focal length. The components are separated by the focal length of the entire lens, which is the same as the focal length of the rear compartment.'''^
A seven-piece objective lens working at //I has been patented. Three of its seven elements are cemented together. ^^