direct your attention to Sir James Dewar's
remarkable result that the photographic plate retains
considerable power of forming the latent image at temperatures
approaching the absolute zero--a result which, as I submit,
compels us to regard the fundamental effects progressing in the
film under the stimulus of light undulations as other than those
of a purely chemical nature. But few, if any, instances of
chemical combination or decomposition are known at so low a
temperature. Purely chemical actions cease, indeed, at far higher
temperatures, fluorine being among the few bodies which still
show

[1] Presidential address to the Photographic Convention of the
United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.

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chemical activity at the comparatively elevated temperature of
-180 deg. C. In short, this result of Sir James Dewar's suggests that
we must seek for the foundations of photographic action in some
physical or intra-atomic effect which, as in the case of
radioactivity or fluorescence, is not restricted to intervals of
temperature over which active molecular vis viva prevails. It
compels us to regard with doubt the role of oxidation or other
chemical action as essential, but rather points to the view that
such effects must be secondary or subsidiary. We feel, in a word,
that we must turn for guidance to some purely photo-physical
effect.

Here, in the first place, we naturally recall the views of Bose.
This physicist would refer the formation of the image to a strain
of the bromide of silver molecule under the electric force in the
light wave, converting it into what might be regarded as an
allotropic modification of the normal bromide which subsequently
responds specially to the attack of the developer. The function
of the sensitiser, according to this view, is to retard the
recovery from strain. Bose obtained many suggestive parallels
between the strain phenomena he was able to observe in silver and
other substances under electromagnetic radiation and the
behaviour of the photographic plate when subjected to
long-continued exposure to light.

This theory, whatever it may have to recommend it, can hardly be
regarded as offering a fundamental explanation. In the first
place, we are left in the dark as to what

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the strain may be. It may mean many and various things. We know
nothing as to the inner mechanism of its effects upon subsequent
chemical actions--or at least we cannot correlate it wit