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TWO SOLUTION PEOCESSING 379
thorough draining to that point, contamination of the newly pumped developer occurs. This is undesirable, since it may result in an undesirable increase in graininess or in other characteristic that cannot be corrected by merely changing the speed of the machine.
In general, the bath that is richest in reducing agents (such as those listed) provides the best photographic quality. To a great degree developing is a matter of how much one can afford to spend on the reducing agents used in the bath. Although increasing the alkali and decreasing the reducing agents in a bath may produce like development contrasts, the major penality is a marked increase in graininess. The price of alkalis is but a fraction of a dollar per pound, whereas reducing agents cost several dollars per pound, and it is not uncommon to find developer baths in commercial laboratories using excess alkali and insufficient reducing agents.*
Printing: The Printer
A printer has been defined as a machine that forms a latent image in the photographic emulsion of raw film with light modulated by an image carried on a separate film. Printers may be classified in accordance with the relationship of the two emulsions concerned: (1) contact — where the emulsion of the film to be copied and the emulsion of the raw stock are in intimate physical contact, and (2) projection — where the image of the film to be copied is projected by suitable optical means upon the emulsion of the raw stock. Projection printers may be classed as : (a) enlarging — where the projected image is larger than the image on the film to be copied; (b) reducing — where the projected image is smaller than the image on the film to be copied; and (c) one-to-one — where the projected image is of the same size as the image on the film to be copied.
A printer requires (1) a light source — often an incandescent lamp, (2) a printer gate or aperture — the portion of the machine where the image is transferred, (3) a light control — this may be an iris diaphragm similar to that used with a camera lens or other aperture control and/or it may be a rheostat connected in the lamp circuit for suitably dimming the lamp, (4) a motor — usually a synchronous a.-c. motor to avoid changes of speed resulting from slight changes of mechanical load, (5) feed and takeup reel arms — for accommodating both the film to be copied and the
* For details as to solution mixing procedure, etc., the reader is referred to Photographic Chemicals and Solutions by Crabtree and Matthews (American Photographic Publishing Company, 1939).