Page:Collected Physical Papers.djvu/229

Rh lastly, it is necessary to have some means of studying all the stages of change in a continuous manner, so that the important preliminary phase of "molecular negotiation" may not be missed. I have in my two previous papers shown how the above ideal requirements may be realised by taking advantage of the conductivity or electromotive variation methods which not only enable the detection of extremely minute molecular changes produced by radiation, but also to follow the changes moment after moment in a continuous manner.

I have described in the two previous papers the various molecular effects induced by light, electric radiation, and mechanical stimulation, under diverse conditions. The consideration of these will give a clear insight into various obscure phenomena connected with photographic action, among which may be mentioned the following:—


 * 1) Photo-chemical induction.
 * 2) Relapse of invisible image.
 * 3) Recurrent reversals.
 * 4) The development of pressure marks.

It is an arbitrary distinction to call a phenomenon either physical or chemical when it happens to be on the common border region. I have shown that when a substance is molecularly strained by light, its chemical activity is modified in consequence of the physical strain. The acted and the unacted portions will therefore be unequally attacked by a developer. In the case of a compound, the strain produced by light may cause a mpdification which renders it susceptible to