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792 Transactions of S.M.P.E., Vol. XII, No. 35, 1928
base. That is, no dirt or other factors giving rise to optical nonhomogeneity are introduced by substrating the base.
Now when the film base is emulsion-coated and put through the regular process of fixing and washing, or developing, fixing, and washing, the level of the noise intensity is found to be somewhat increased. The question naturally arises: Is the gain in noise due to the emulsion,' or is it due to the processing? To answer this question two experiments were carried out. One was to find out the difference in the noise level introduced on two pieces of the same film in wiping the film after washing by machine or by hand. The results obtained showed that the noise level on the film which has received careful manual wiping was less than half that which had been wiped by machine. These results clearly indicate the importance of very careful handling during processing film used for sound recording. The second experiment was to take a piece of film base, substrated but not emulsion-coated, and run it through the regular process of developing, fixing, washing, and drying. The results obtained showed that noise level had been increased considerably above that of another sample of the same piece of film which had not been processed, and, furthermore, that it was comparable in magnitude to the noise level on a film emulsioncoated and processed. It may be concluded, therefore, that there is no appreciable increase in the noise level at the various stages of manufacture. There is, however, a considerable increase in the noise level during the process of developing, fixing, washing, and drying even when these operations are carried out by a skilled worker and with the utmost care.
It is of interest to note that film fixed without development is noisier than film which has been developed so as to add a small density (about 0.1) due to a certain amount of fog present; and that the film which has received a uniform exposure so as to increase this density to about 0.25 has still less noise. This can be accounted for on the basis that a certain amount of dirt, speaking in a broad sense, has been introduced, and that the presence of this dirt causes a larger variation in the light incident on the photo-electric cell on the clear film than on one of which the transmission has been reduced somewhat by a uniform silver deposit.
It seemed to be of interest to determine how the noise would grow by repeatedly running the film through the projector. To investigate this point four samples of film were prepared, two