shows the effect of light of a given intensity. It will be noticed that light makes the acted wire cuproid. But the action of mechanical vibration (see lower curve in same figure) makes the acted wire zincoid, and after several trials I found that a vibration with an amplitude of 3° produced a series of curves similar, but of opposite sign, to those produced by light. Thus mechanical vibration produced a molecular effect opposite to that of light.
I next allowed both the disturbing influences to act simultaneously on one of the wires, and the light action was then found to be exactly balanced by the action of mechanical vibration, an increase or diminution of either at once upsetting the balance.
The molecular effect of mechanical vibration thus appears, at least in the case of tin, to be opposite to that produced by light. This may be the case in general: the exception might be when one of the two stimuli is normal and the other sub-normal.
Our uncertainty with regard to the correct theory of photographic action is due to great experimental difficulties in studying the problem. As for instance:—
(1.) There is reason to believe that every substance is molecularly affected by radiation, but detection of change is rendered impossible by the imperfections of methods hitherto available, and also by the subsequent self-recovery of the substance in darkness. The effects can be detected in a few cases only when the changes produced happen to be visible, or become visible on development.
(2.) As regards direct chemical tests, if we take, for example, the case of AgCl, the quantity of radiation product is exceedingly small, and occurs in the presence of a very large amount of unchanged chloride. The isolation of the minute traces of changed product has baffled all effort. Again, there are produced various secondary reactions which complicate the phenomenon.
To arrive at a correct idea of the changes produced, it is necessary to measure the minute effects produced by radiation on the extremely thin layer—perhaps only a few molecules deep—of the sensitive substance. In order to ascertain this, it is desirable to begin with the study of some elementary substance in which its effects are attended with few secondary complications. And, lastly, it is necessary to have some means of studying all the stages of change in a continuous
