1901.] On the Strain Theory of Photographic Action. 187 



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 linage. 



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 sen- 

 sitive to radiation, and yet seem to show no effect capable of photo- 

 graphic 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. 



