If sensitive areas exist in unexposed crystals, the number of development centers formed on exposure is given by the Poisson law, and this conflicts with experiment. But if a development center is formed in a diatomic reaction triggered by P photons, the fraction of the grains n capable of development after an exposure E is
and this fits the grain counts of Slade, Toy, and Sheppard.
The photon number P increases with the size of the crystal. Nonintegral values of P cannot be attributed to a mixture of grains each with P integral, but are inherent in the constitution of a single grain.
The reaction cross section s can be accurately calculated, provided the exposure has been measured in absolute units, for the theory of dimensions requires that E be expressed in photons per unit area. In two emulsions examined s is proportional to the volume of the crystals, and in a third to the surface area.
If a development center is held to consist of two silver atoms, the reason why the two photons absorbed must be contiguous in space and nearly coincident in time is manifest, and the accounts of fertilization and reciprocity failure achieve a new unity.
© 1962 Optical Society of AmericaFull Article | PDF Article
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