[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 with what is known of the physics of chemical activity.  Finally, as will be seen later, it is hardly adequate to account for the varying degrees of stability which may apparently characterise the latent image.  Still, there is much in Bose’s work deserving of careful consideration.  He has by no means exhausted the line of investigation he has originated.

Another theory has doubtless been in the minds of many.  I have said we must seek guidance in some photo-physical phenomenon.  There is one such which preeminently connects light and chemical phenomena through the intermediary of the effects of the former upon a component part of the atom.  I refer to the phenomena of photo-electricity.

It was ascertained by Hertz and his immediate successors that light has a remarkable power of discharging negative electrification from the surface of bodies—­especially from certain substances.  For long no explanation of the cause of this appeared.  But the electron—­the ubiquitous electron—­is now known with considerable certainty to be responsible.  The effect of the electric force in the light wave is to direct or assist the electrons contained in the substance to escape from the surface of the body.  Each electron carries away a very small charge of negative electrification.  If, then, a body is