Page:On the Strain Theory of Photographic Action.djvu/5

1901.]

I have previously remarked that the molecular strain curve in general is in the first part slightly convex, then straight, and in the last part concave; this is true not only when the strain is produced by light, but also by mechanical vibration.

(2.) After this stage, the curve of response rises almost in a straight line. This is the stage of increasing action.

(3.) The curve then reaches the maximum and becomes horizontal; after which it begins to fall, till it reaches the original neutral line.

(4.) After very prolonged exposure I have sometimes found the curve proceeding in the negative direction, thus exhibiting molecular reversal.

I have before explained the similarities of the molecular strains produced by light and mechanical vibration. The recurrent reversals are also sometimes obtained with mechanical vibration, as in the following electromotive variation curve for nickel (see fig. 18), which was kept for a long time under constant mechanical vibration.

The first part of the curve, or the latent period, is very suggestive as regards the obscure phenomenon of photo-electric induction. Thus "Quantitative measurements have shown that the action of light is not instantaneous. On the contrary, it gradually develops, and requires a considerable time before it attains its full strength. When a mixture of chlorine and hydrogen, which have been kept in the dark, is exposed VOL. LXX.