riment. 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

209

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 fi