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April, 1930] EXHAUSTION OF DEVELOPERS 391
velopment starts is responsible for the loss of image density, then, if development is started in a fresh developer and completed in an exhausted developer, no shadow detail should be lost. This possibility was tested carefully using a fresh borax developer and a badly exhausted one. The latter had been used in a large tank for developing motion picture negative film, and it had been revived after 80 feet per gallon. When the tank was finally discarded after a total of 160 feet per gallon, a small sample was exhausted further. When Eastman motion picture panchromatic negative film was developed for eight minutes in the fresh and ten and one-half minutes in the revived and exhausted developer a gamma of 0.7 was obtained in both developers. The film developed in the exhausted developer, however, appeared to have only half the exposure of that developed in the fresh developer although actually the strips had been exposed exactly alike.
Tests were then made by starting strips for differing lengths of time in the fresh developer and completing development to the same gamma in the old developer. These tests showed that, if the film were developed for 2 minutes in the fresh developer followed by §l/z minutes in the old developer, the same gamma of 0.7 could be obtained with no loss in exposure, although both fog and image density were slightly less than with the fresh developer. With 4 minutes in the fresh and 4 minutes in the old, the total density also was the same as with 8 minutes in the fresh. The exact times stated above applied to the particular developers, and emulsion used in these tests would vary under different conditions.
In a developing machine where fresh developer is put in continuously at one end and the exhausted developer is allowed to overflow at the other, always maintaining constant development conditions, it is obvious from the above data that the fresh borax developer should be introduced where the film enters and moved through the machine in the same direction as the film. This is opposite to what would be done with most developers as development would ordinarily be started in the seasoned portion of the developer in order to prevent fog.
THE DEVELOPER TEST
The developer test described herein is of a photographic nature and is based on the well-known principle of the Watkins factor. Instead of determining time of appearance of an image, however, it