1901.] On the Strain Theory* of Photographic Action. 189 

 +'0/oV.i — 



Time. 



10 IE 14 I6mms. 



Fig. 17. — E.M. variation curve for AgBr cell, under the continued action of light. 

 Note the preliminary negative twitch. 



I have previously remarked that the molecular strain curve in 

 general is in the first part slightly convex, then straight, and in the 

 last part concave ; this is true not only when the strain is produced 

 by light, but also by mechanical vibration. 



(2.) After this stage, the curve of response rises almost in a straight 

 line. This is the stage of increasing action. 



(3.) The curve then reaches the maximum and becomes horizontal ; 

 after which it begins to fall, till it reaches the original neutral line. 



(4.) After very prolonged exposure I have sometimes found the 

 curve proceeding in the negative direction, thus exhibiting molecular 

 reversal. 



I have before explained the similarities of the molecular strains 

 produced by light and mechanical vibration. The recurrent reversals 

 are also sometimes obtained with mechanical vibration, as in the 

 following electromotive variation curve for nickel (see fig. 18), which 

 was kept for a long time under constant mechanical vibration. 



.02V, 





Mi 











t 















\ry 







Time. 



Fig. 18. — Recurrent reversals obtained with a M cell under continued vibration. 



6. Photo-chemical Induction. 



The first part of the curve, or the latent period, is very suggestive 

 as regards the obscure phenomenon of photo-electric induction. Thus 

 " Quantitative measurements have shown that the action of light is not 

 instantaneous. On the contrary, it gradually develops, and requires a 

 considerable time before it attains its full strength. When a mixture 

 of chlorine and hydrogen, which have been kept in the dark, is exposed 



VOL. LXX. 



