In a previous investigation carried out with a contrast motion-picture positive emulsion of uniform grain size, it was found that for the first few minutes of development in a standard Elon-hydroquinone developer the differential density decrease produced by the Herschel effect was constant in each exposure step of the pre-exposure to white light. This led to the conclusion that the “critical size” of the development centers remained essentially unchanged during that time.
In the present investigation with the same emulsion, the constancy of the Herschel effect was confirmed, both with the same developer and with a pure surface ferrous oxalate developer. When development baths were used which contained varying amounts of sodium thiosulfate, the Herschel effect decreased, corresponding to the solvent action. This agrees with the widely accepted view that the Herschel effect produces a redistribution of centers through the grain.
The Herschel effect proved also to be a sensitive indicator of the growth of surface centers which occurred when white-light and infra-red exposures were separated by longer time intervals. When the film was kept at higher temperatures, the decrease of the Herschel effect was the only evidence of latent-image change. Furthermore, the very great difference in the Herschel effect corresponding to different intensity levels of pre-exposure gave a striking illustration of the difference in center size produced at these different intensity levels.
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