r conditions) when
exposed to ultra-violet light. Photographers will recognise that
this is also the order of their photographic sensitiveness.

Another class of bodies also concerns our subject: the special
sensitisers used by the photographer to modify the spectral
distribution of sensibility of the haloid salts, _e.g._ eosine,
fuchsine, cyanine. These again are electron-producers under light
stimulus. Now it has been shown by Stoletow, Hallwachs, and
Elster and Geitel that there is an intimate connection between
photo-electric activity and the absorption of light by the
substance, and, indeed, that the particular wave-lengths absorbed
by the substance are those which are effective in liberating the
electrons. Thus we have strong reason for believing that the
vigorous photo-electric activity displayed by the special
sensitisers must be dependent upon their colour absorption. You
will recognise that this is just

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the connection between their photographic effects and their
behaviour towards light.

There is yet another suggestive parallel. I referred to the
observation of Sir James Dewar as to the continued sensitiveness
of the photographic film at the lowest attained extreme of
temperature, and drew the inference that the fundamental
photographic action must be of intra-atomic nature, and not
dependent upon the vis viva of the molecule or atom. In then
seeking the origin of photographic action in photo-electric
phenomena we naturally ask, Are these latter phenomena also
traceable at low temperatures? If they are, we are entitled to
look upon this fact as a qualifying characteristic or as another
link in the chain of evidence connecting photographic with
photo-electric activity.

I have quite recently, with the aid of liquid air supplied to me
from the laboratory of the Royal Dublin Society, tested the
photo-sensibility of aluminium and also of silver bromide down to
temperatures approaching that of the liquid air. The mode of
observation is essentially that of Schmidt--what he terms his
static method. The substance undergoing observation is, however,
contained at the bottom of a thin copper tube, 5 cm. in diameter,
which is immersed to a depth of about 10 cm in liquid air. The
tube is closed above by a paraffin stopper which carries a thin
quartz window as well as the sulphur tubes through which the
connections pass. The air within is very carefully dried by
phosphorus

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pentoxide before the expe