The Herschel effect, on investigation at different density steps and with varying times of irradiation, proves to be a reaction which starts in each development center upon the absorption of one quantum. At low initial densities, a large number of the developable grains revert to the undevelopable state upon the removal of one or two silver atoms per center. Results obtained at different times of development in a standard hydroquinone-elon developer point to the conclusion that the “critical size” of the development centers remains essentially unchanged during the first few minutes of development, while at longer times of development centers of smaller size contribute to the developed density.
G. KORNFELD and T. H. JAMES, "The Herschel Effect in Relation to a Critical Size of Development Centers," J. Opt. Soc. Am. 33, 615-618 (1943)