In the above case, it is difficult to see how light could have produced any chemical action in the relatively inactive silver salt. It is more likely that the effect produced was of the nature of some physical strain. That light does produce molecular strain even at such low temperatures—a strain which may remain latent—is shown from Professor Dewar's experiments on phosphorescence. Ammon. Pt. Cyanide cooled to - 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.
2. Substances may be Sensitive and yet give no Photographic Image.
The photographic effect on a sensitive plate is usually demonstrated by appropriate development, long after the 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 due to radiation; for otherwise there would be nothing to distinguish the light-impressed portions from the other portions 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 the exposure will not be successful in cases where there is quick self-recovery. It will only be successful where the strain effect is more or less permanent.
It is thus seen that it is quite possible for a substance to be sensitive to radiation, and yet seem to show no effect capable of photographic development, owing to rapid self-recovery.
3. Relapse of the Invisible Image.
The above considerations afford a simple explanation of the very obscure phenomenon of the relapse of the invisible image. Recovery is merely a question of time. With certain substances it is immediate, with others it takes a little longer, as in a daguerreotype, where the latent image only disappears in the course of several hours. In ordinary photographic plates the recovery may not take place for several years. We have seen how the strain effect of electric radiation was transient in some cases, whereas it was persistent in others.
