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Work has been continued on the failure of the photographic reciprocity law, E=It. The failure of this law using Panchromatic and Orthochromatic emulsions is somewhat greater than reported previously for high speed non-color sensitive emulsions. Slow Ordinary and Lantern emulsions show very great failure and also a marked change of gamma with intensity. In general, for fast plates the variation of density with intensity is small, optimal intensity is at a low value, and gamma is independent of intensity. For slow plates the density variation is great, optimal intensity high, and gamma drops at low intensities. As found by Kron, optimal intensity shifts to higher values with increasing development time.
Kron’s usually accepted empirical law does not fit the observed data. An alternative form
has been found to fit the observed data for fast plates over a range of 1:8,000,000 in intensity and to fit the data for slow plates at high intensities. For these emulsions the observed failure is greater than the calculated curve indicates at low intensities. This equation was suggested by Kron but it did not fit his observations. The observed data thus far indicate that the dependence of gamma on intensity follows as a result of changes in the effective grain size frequency function caused by a differential failure of the reciprocity law for grains of different sizes and sensitivities.© 1926 Optical Society of America
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