"Angel, I can go with thee no farther. Is there, then, no end to the universe of stars?" [1] De Quincy in his _System of the Heavens_ gives a fine paraphrase of "Richter's Dream." 200 Then the angel flung up his glorious hands to the heaven of heavens, saying "End is there none to the universe of God? Lo! also there is no beginning." 201 THE LATENT IMAGE [1] My inclination has led me, in spite of a lively dread of incurring a charge of presumption, to address you principally on that profound and most subtle question, the nature and mode of formation of the photographic image. I am impelled to do so, not only because the subject is full of fascination and hopefulness, but because the wide topics of photographic methods or photographic applications would be quite unfittingly handled by the president you have chosen. I would first 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. 202 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.