Before proceeding to notice the early efforts of those who laid the foundation of the art of photography, with which we are now to be chiefly engaged, we may observe that Priestley, Senebier, Ingenhousz, De Candolle, Soussure, and Ritter, directed attention to the influence of light upon plants—an interesting and important subject. Others followed in a similar track, still, however, leaving the matter in a com paratively obscure condition. The action of light on the human frame, and on animal life generally, has not yet been fairly investigated. That some special action will be detected there can be no doubt. We have long thought that light will come to be considered as impor tant an element to health as fresh air and wholesome food. It may possibly be that much mental or bodily labour, exercised in the absence of the stimulus of daylight, is indirectly injurious to animal life.
But let us proceed to trace rapidly the art of photography to its source. In the Journals of the Royal Institution of Great Britain for 1802 will be found a paper by " Thomas Wedgwood and Hum phry Davy"—the first a brother of the famous porcelain manufacturer, the second the Sir Humphry Davy of a later period. Their joint paper was entitled ' An Account of a Method of Paintings upon Glass and of making Profiles by the Agency of Light upon Nitrate of Silver ; with Observations by II. Davy.' This paper con tains the complete germs of the photographic art, namely, the applica tion of an optical iustrument to imprint upon a sensitive chemical surface the images of all natural objects illuminated by the sunbeam, or other source of light. The instrumenta used by them observers were the camera obscure said the solar niieroecope but let us first see bow this important invention took shape in the mind of NVedgwood.
According to Davy, Wedgwood first oonnuenced his remerches with a view to copy the images of the camera obscure; and for this pur pose, says Davy, "he fiat used nitrate of silver, which was mentioned to him by a friend as a substance very sensible to the influence of light." This nitrate of silver was applied in solution to surfaces of white paper and loather. "White paper or white leather," to quote the words of the memoir of 1802, " moistened with solution of nitrate of silver, undergoes no change when kept in a dark place; but on being exposed to the daylight, it speedily changer colour, after paining through different shades of grey and brown, becomes at length nearly black. The alterations of colour take place more
speedily in proportion as the light is more intense. In the direct beeni of the sun two or three minutes are sufficient to produce the full effect; in the shade several hours are required; and light trans mitted through different-coloured glasses sets upon it with different degrees of intensity." " When the shadow of any figure is thrown upon the prepared surface, the part concealed by it remains white, and the other parts speedily become dark. For copying paintings on glass the solution should be applied on leather, and in this case it is more readily acted on than when paper is used. After the colour has been once fixed on the leather or paper, it cannot be removed by the appli cation of water, or water and seep ; and it is in a high degree per manent. The copy of a painting, or the profile, immediately after being taken must be kept In an obscure place; it may, indeed, be examined in the ahaelo, but in this cam the exposure should be only for a few minutes : by the light of caladium or lamps, as commonly employed, it is not sensibly affected." No means were found to fix permanently the impressions thus produced. And, as regard. the primary end of Wedgwood's researches, we are told that " The images formed by means of a camera obscure have been found to be too faint to produce in any moderate time an effect upon the nitrate of silver." Davy adds, "In following these processes, I have found that the images of small objects produced by means of the solar microscope, may be copied without difficulty on prepared paper. This will pro bably be a useful application of the method; that it may be employed successfully, however, it is necessary that the paper be placed at but a small distance from the lens." The muriate (chloride) of silver was found to be more sensitive to light than the nitrate. " Even in the twilight, the colour of the moist muriato of silver spread upon paper slowly changed from white to faint violet ; though under similar circumstances no immediate alte ration was produced upon the nitrate." Davy concludes with these remarkable words : "Nothing but a method of preventing the un shaded parts of the delineations from being coloured by exposure to the day, is wanting to render this process as useful as it is elegant." From this time the art in England slumbered until 1834, when Mr. Fox Talbot, without knowing what had been done, commenced experiments with the same end in view.