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ionisation constituting the latent image, where the ion is probably not immediately neutralised by chemical combination, presents features akin to the charging of a capacity—­say a Leyden jar.  There may be a rising potential between the groups of ions until ultimately a point is attained when there is a spontaneous neutralisation.  I may observe that the phenomena of reversal appear to indicate that the change in the silver bromide molecule, whatever be its nature, is one of gradually increasing intensity, and finally attains a maximum when a return to the original condition occurs.  The maximum is the point of most intense developable image.  It is probable that the sensitiser—­in this case the gelatin in which the bromide of silver is immersed—­plays a part in the conditions of stability which are involved.

Of great interest in all our considerations and theories is the recent work of Wood on photographic reversal.  The result of this work is—­as I take it—­to show that the stability of the latent image may be very various according to the mode of its formation.  Thus it appears that the sort of latent effect which is produced by pressure or friction is the least stable of any.  This may be reversed or wiped out by the application of any other known form of photographic stimulus.  Thus an exposure to X-rays will obliterate it, or a very brief exposure to light.  The latent image arising from X-rays is next in order of increasing stability.  Light action will remove this.  Third in order is a very brief light-shock or sudden flash.  This

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cannot be reversed by any of the foregoing modes of stimulation, but a long-continued undulatory stimulus, as from lamp-light, will reverse it.  Last and most stable of all is the gradually built-up configuration due to long-continued light exposure.  This can only be reversed by overdoing it according to the known facts of recurrent reversal.  Wood takes occasion to remark that these phenomena are in bad agreement with the strain theory of Bose.  We have, in fact, but the one resource—­the allotropic modification of the haloid—­whereby to explain all these orders of stability.  It appears to me that the elasticity of the electronic theory is greater.  The state of the ionised system may be very various according as it arises from continued rhythmic effects or from unorganised shocks.  The ionisation due to X-rays or to friction will probably be quite unorganised, that due to light more or less stable according to the gradual and gentle nature of the forces at work.  I think we are entitled to conclude that on the whole there is nothing in Wood’s beautiful experiments opposed to the photo-electric origin of photographic effects, but that they rather fall in with what might be anticipated according to that theory.

When we look for further support to the views I have laid before you we are confronted with many difficulties.  I have not as yet detected any electronic discharge from the film under light stimulus.  This may be due to my defective experiments, or to a fact noted by Elster and Geitel concerning the photo-electric properties of gelatin.