Page:Collected Physical Papers.djvu/403

Rh 4. The ascending portion of the curve of response is abrupt, whereas the fall during recovery is at first rapid and then comparatively slow; the curve of recovery is thus convex to the abscissa which represents time.

5. Under increasing intensity of stimulation, the amplitude of response undergoes an increase which reaches a limit.

6. Sub-minimal stimulus induces a response of opposite sign to that under moderate stimulation.

7. Under rapidly succeeding stimuli, the individual responses become fused; the curve rises to a maximum, when the force of restitution balances the force of distortion due to stimulation.

8. Under prolonged stimulation, the response tends to become reversed; during this process the reversal may become recurrent.

The photographic effect of light is detected in a few cases only, when the induced change happens to be visible or is rendered visible on development; the image often disappears on account of self-recovery during darkness. The changes induced can, however, be followed in all its phases in the resulting curve of electric response. In this curve the period of overcoming molecular inertia corresponds to the induction period of photographic action; the automatic recovery explains relapse of the impressed image.

This impression can be rendered more permanent by molecular overstrain under strong and long-continued action of light, the recovery from overstrain being thus greatly prolonged. The fact that molecular strain induced by light is universal, is shown by images