Page:Collected Physical Papers.djvu/231

Rh −180° C. in liquid air absorbs light, but emits feeble radiation. But as the temperature is raised the stored-up light is emitted with very great intensity.

I now proceed to consider the photographic interpretations of the various molecular response curves taken under the action of radiation, as detailed in my previous papers.

The photographic effect on a sensitive plate is usually demonstrated by appropriate development, long after exposure. The after-effect of light on the sensitive substance may be fugitive or persistent. There are numerous gradations of this persistency of after-effect.

In order that the effect of light may be "developed," it is therefore necessary that the portions corresponding to the image should not in the meantime have recovered from the strain induced by radiation; for otherwise there would be nothing to distinguish the light-impressed portions from others not affected by light.

Though almost all substances are molecularly affected by radiation, yet there is a great difference in the permanence of after-effects. The recovery, as has been mentioned before, is very quick in some cases, whereas in others it may be protracted.

It is obvious that any method which attempts to develop the after-effect a long time after exposure will not be successful in cases where there is a quick self-recovery. It will only be successful where the strain effect is more or less persistent.

It is thus seen that it is quite possible for a substance to be sensitive to radiation, and yet seem to show no