188 



Prof. J. C. Bose. 



[Apr. 18, 



It is evident that in order to make the after-effect more or less 

 permanent, and thus render it developable, self-recovery should be 

 retarded. There are two ways in which the after-effect may be 

 rendered comparatively permanent : (1) Even a highly elastic sub- 

 stance may be rendered more or less permanently distorted by 

 straining it beyond the limit of elastic recovery ; or (2) the presence 

 of a retarding substance may prevent the self-recovery of the sensitive 

 material. One of the chief functions of the so-called sensitisers may 

 be to prevent self-recovery and make the after-effect permanent. 



4. Permanence of the Affect-effect by Overstrain. 



Thus in many cases where images cannot be obtained with ordinary 

 exposure, they can be obtained with excessive strain caused by pro- 

 longed exposure. Thus Moser obtained an invisible image on a clean 

 silver plate by exposing it to the sun for 2 hours or more. The 

 invisible image was afterwards fixed by development with mercury 

 vapour. A similar result was obtained with copper. 



Major-General Waterhouse describes a very interesting series of 

 investigations* in which by prolonging the exposure, printing-out 

 impressions were obtained on silver. These could be developed not 

 only by mercury or water vapour, but also by ferrous sulphate or 

 pyrogallic developers. Images were also obtained on lead and gold. 



All these results derive an additional interest from the fact that 

 most of the phenomena that occur by the exposure of ordinary photo- 

 graphic plates containing haloid compounds of silver can also be 

 observed upon a silver plate exposed to light. In my experiments on 

 molecular effects produced by electric waves, I found all metals sensitive 

 to electric radiation, owing to the extremely delicate nature of the 

 conductivity method of detection. The molecular effects of visible 

 radiation on various substances are also exhibited by the electromotive 

 variation method. In the experiments of Waterhouse, a considerable 

 number of metals were found to be sensitive to visible radiation, the 

 effect being rendered more or less permanent by overstrain. 



5. Electromotive Variation Curve due to Light. 



I give below one out of several similar curves, showing the effect of 

 continuous light on one of the two plates in a photo-electric cell of AgBr 

 (see fig. 17). In this curve several distinct stages are noticeable. 



(1.) A short latent period, where there is apparently little or no 

 action or even a transitory negative action. The curve given had to 

 be contracted to put in all the different phrases, and the peculiarities 

 of the first part cannot be very well shown. 



* Waterhouse, ' Eoy. Soc. Proc.,' April, 1900. 



