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pentoxide before the experiment.  The arc light is used as source of illumination.  It is found that a vigorous photo-electric effect continues in the case of the clean aluminium.  In the case of the silver bromide a distinct photo-electric effect is still observed.  I have not had leisure to make, as yet, any trustworthy estimate of the percentage effect at this temperature in the case of either substance.  Nor have I determined the temperature accurately.  The latter may be taken as roughly about -150 deg.  C,

Sir James Dewar’s actual measilrements afforded twenty per cent. of the normal photographic effect at -180 deg.  C. and ten per cent. at the temperature of -252.5 deg.  C.

With this much to go upon, and the important additional fact that the electronic discharge—­as from the X-ray tube or from radium—­generates the latent image, I think we are fully entitled to suggest, as a legitimate lead to experiment, the hypothesis that the beginnings of photographic action involve an electronic discharge from the light-sensitive molecule; in other words that the latent image is built up of ionised atoms or molecules the result of the photo-electric effect on the illuminated silver haloid, and it is upon these ionised atoms that the chemical effects of the developer are subsequently directed.  It may be that the liberated electrons ionise molecules not directly affected, or it may be that in their liberation they disrupt complex molecules built up in the ripening of the

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emulsion.  With the amount we have to go upon we cannot venture to particularise.  It will be said that such an action must be in part of the nature of a chemical effect.  This must be admitted, and, in so far as the rearrangement of molecular fabrics is involved, the result will doubtless be controlled by temperature conditions.  The facts observed by Sir James Dewar support this.  But there is involved a fundamental process—­the liberation of the electron by the electric force in the light wave, which is a physical effect, and which, upon the hypothesis of its reality as a factor in forming the latent image, appears to explain completely the outstanding photographic sensitiveness of the film at temperatures far below those at which chemical actions in general cease.

Again, we may assume that the electron—­producing power of the special sensitiser or dye for the particular ray it absorbs is responsible, or responsible in part, for the special sensitiveness it confers upon the film.  Sir Wm. Abney has shown that these sensitisers are active even if laid on as a varnish on the sensitive surface and removed before development.  It must be remembered, however, that at temperatures of about -50 deg. these sensitisers lose much of their influence on the film; as I have pointed out in a paper read before the Photographic Convention of 1894.

It. appears to me that on these views the curious phenomenon of recurrent reversals does not present a problem hopeless of explanation.  The process of photo-