Page:Collected Physical Papers.djvu/195

Rh suggestive in regard to a not altogether different after-effect of light in the fluorescent and phosphorescent bodies. In the case of Ag′ receiver, owing to its molecular mobility, the recovery is automatic. But in the case of so-called non-recovering substances, the strain persists for a considerable time; the recovery can, however, be hastened by removing molecular friction through gentle heating. In connection with this, I will quote an interesting observation previously made. In an iron receiver strained by radiation there was quick recovery after heating, and careful inspection showed a slight oscillatory movement of the galvanometer spot during the process. Here the strain produced by radiation remained latent to be released by heat. In the phenomenon of thermo-luminescence, the strain effect of light also remains latent till set free by the application of heat.

Effect of Continuous Radiation.—Still more interesting are the superposed effects of a series of flashes of radiation. The first flash produces a certain molecular distortion, attended with conductivity variation, from which it tends to recover. This force of restitution will be shown to increase with increasing distortion. Now if, before the substance has recovered from the first shock, a second flash be superposed, it will produce further distortion; but the effect will not be quite so strong, inasmuch as the force of restitution is increasing. Thus a series of superposed flashes will produce a limiting effect, which is kept balanced by the force of restitution. Under increased intensity of radiation, the balancing position is at a higher level. The after-effect, moreover, is more persistent under stronger intensity of stimulation.