1901.] On the Strain Theory of Photographic Action. 185 



shows the effect of light of a given intensity. It will be noticed that 

 light makes the acted wire cuproid. But the action of mechanical 

 vibration (see lower curve in same figure) makes the acted wire zincoid^ 

 and after several trials I found that a vibration with an amplitude of 

 3° produced a series of curves similar, but of opposite sign, to those 

 produced by light. Thus mechanical vibration produced a molecular 

 effect opposite to that of light. 



I next allowed both the disturbing influences to act simultaneously 

 on one of the wires, and the light action was then found to be exactly 

 balanced by the action of mechanical vibration, an increase or diminu- 

 tion of either at once upsetting the balance. 



The molecular effect of mechanical vibration thus appears, at least 

 in the case of tin, to be opposite to that produced by light. This may 

 be the case in general : the exception might be when one of the two 

 stimuli is normal and the other sub-normal. 



" On the Strain Theory of Photographic Action." By Jagadis 

 Chunder Bose. Communicated by Lord Eayleigh, F.K.S. 

 Eeceived April 18,— Eead June 20, 1901. 



Our uncertainty with regard to the correct theory of photographic 

 action is due to great experimental difficulties in studying the problem. 

 As for instance : — 



(1.) There is reason to believe that every substance is molecularly 

 affected by radiation, but detection of change is rendered impossible 

 by the imperfections of methods hitherto available, and also by the. 

 subsequent self-recovery of the substance in darkness. The effects can 

 be detected in a few cases only when the changes produced happen to 

 be visible, or become visible on development. 



(2.) As regards direct chemical tests, if we take, for example, the 

 case of AgCl, the quantity of radiation product is exceedingly small, 

 and occurs in the presence of a very large amount of unchanged 

 chloride. The isolation of the minute traces of changed product has 

 baffled all effort. Again, there are produced various secondary reac- 

 tions which complicate the phenomenon. 



To arrive at a correct idea of the changes produced, it is necessary 

 to measure the minute effects produced by radiation on the extremely 

 thin layer — perhaps only a few molecules deep — of the sensitive sub- 

 stance. In order to ascertain this, it is desirable to begin with the 

 study of some elementary substance in which its effects are attended 

 with few secondary complications. And, lastly, it is necessary to 

 have some means of studying all the stages of change in a continuous 



