1901.] On the Strain Theory of Photographic Action. 



193 



trie Touch "* I have given similar instances of reversals produced by 

 the action of long-continued electric radiation. 



10. Other Methods of obtaining Latent Image. 



If molecular strain be the basis of all photographic phenomena, then 

 it ought to be possible to obtain latent images by other methods of 

 producing molecular strain. 



An instance of this is seen in the development of mechanical 

 pressure marks. Images produced by electric strain are seen in the 

 " inductoscripts." 



11. Conclusion, 



It is thus seen — 



(1.) That molecular strain is produced by the action of light. 



(2.) That as the physico-chemical properties of a substance are 

 changed by strain, it is possible to develop the latent image through 

 the difference in the following properties between the exposed and 

 unexposed portions produced by light — 



(a.) Difference in adhesive power, e.g., development of daguerreo- 

 type by mercury vapour, development by water vapour. 



(b.) Difference in chemical stability, e.g., development by reducing 

 agents. 



(3.) That molecular strain may not only be produced by visible or 

 invisible radiation, but also by (a) electric induction, (b) mechanical 

 distortion. Latent images produced by such means may be developed, 

 e.g., inductoscripts, development of pressure marks. 



(4.) That nearly all substances are sensitive to radiation, but the 

 effect cannot in all cases be rendered visible, (a) owing to want of suit- 

 able chemical developers, (b) owing to quick self-recovery. The mole- 

 cular effect due to radiation can, however, be demonstrated by the 

 conductivity or electromotive variation methods. 



(5.) That the latent period of overcoming inertia corresponds to the 

 photographic induction period. 



(6.) That the relapse of image is due to self-recovery. 



(7.) That owing to the tendency towards self -recovery the radiation 

 effect does not solely depend on the total quantity of light, but 

 depends also on the time-rate of illumination. Hence the photo- 

 graphic effects of intermittent and continuous illuminations are not 

 the same. 



(8.) That the continuous action of radiation produces recurrent 

 reversals. 



(9.) That the molecular effects produced by light and electric 

 radiation are similar. 



* 'Roy. Soc. Proc.,' vol. 66. 



VOL. LXX. 



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