Page:Collected Physical Papers.djvu/233

Rh 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.

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 sensitive method of conductivity variation. The molecular effects of visible radiation on various substances are also exhibited by the method of electromotive variation. 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.

I give below one out of several similar curves, showing the effect of continuous light on one of the two plates of a photo-electric cell of AgBr (fig. 48). In this curve several distinct stages are noticeable.