The Failure of the Photographic Reciprocity Law at Low Intensity*

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Abstract
A study of the failure of the reciprocity law at low intensity in photographic exposure was carried out by means of a series of equal energy exposures made partly with high intensity and partly with low intensity illumination. The high and low intensity parts were given in different time order. The results indicate that the failure of the reciprocity law at low intensity is due to a regression of the latent-image speck in its initial stages of formation, while it is still small. If the speck is built up to a stable size by a primary high intensity exposure, then subsequent exposure at low intensity can continue to build up the speck as efficiently as exposure at high intensity. The results show that there can exist a latent-image speck, which is stable, but which is not yet developable.

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