Page:On the Strain Theory of Photographic Action.djvu/2

186 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.

These methods not only enable us to detect 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 produced by light, electric radiation, and mechanical disturbance under different 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.

1. "Chemical" and "Physical" Theories of Photographic Action.

It is an arbitrary distinction to call a phenomenon either physical or chemical when it happens to be on the common borderland. 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 modification which renders it susceptible to decomposition by the action of a reducing agent. The observed evolution of chlorine when moist AgCl is exposed to the long-continued action of intense light is often adduced in support of the chemical nature of photographic action. This extreme case of dissociation cannot, however, be regarded as representative of the action of light in the formation of latent images. In ordinary photographic action we have merely the effect of a moderate stress producing the corresponding strain (with concomitant variation of chemical activity), and not the disruptive effect of a breaking stress.

With reference to photographic action, various facts are known which cannot be well explained from purely chemical considerations. In connection with this the following experiment of Professor Dewar is suggestive. It is found that at the low temperature of —180° C., there is a cessation of all chemical action. Even such an extremely active substance as K does 11ot show any action when immersed in liquid oxygen. Now at these extremely low temperatures, where the action of such an active substance as K is suspended, an Eastman film was still found fairly sensitive to photographic influence.