Page:On the Strain Theory of Photographic Action.djvu/4

188 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 substance 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. Permanance of the After-effect by Overstrain.

Thus in many cases where images cannot be obtained with ordinary exposure, they can be obtained with excessive strain caused by prolonged 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 photographic 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.