A statement has been made by the French, to the effect that M. Charles was in possession of a process by which portraits could be obtained by the agency of sunlight, producing a dark impression upon a prepared surface. This is, however, exceedingly doubtful, and even the Abbé Moigno, in his Répertoire, states, that M. Charles never disclosed any fact connected with this discovery, and that he left no evidence behind him of being in possession of such a process.
In addition to the interesting: facts alreadv mentioned, it will be instructive to add a few particulars of other inquiries pursued about the same time on various phenomena connecting themselves with the solar radiations. Although these do not bear directly on Photography, they stand in very close relation with it, and will serve possibly to indicate lines of research which have not been fully followed out.
Desmortiers in 1801 published a paper in Gilbert's Annals, entitled, " Recherches sur la Decoloration spontanée du Bleu de Prusse," subsequently translated into Nicholson's Journal, in which he has mentioned the influences of the solar rays in producing the change. Böckman about the same time observed that the two ends of the spectrum acted differently on phosphorus; and Dr. Wollaston, examining the chemical action of the rays of the spectrum, arrived at nearly the same results as Bitter. He states, " This and other effects usually attributed to light are not in fact owing to any of the rays usually perceived".
Wedgwood was certainly the first person who made any attempts to use the sunbeam for delineating the objects it illuminated: it is therefore necessary that some more particular account should be given of his processes. In 1802, he published a paper in the Journal of the Royal Institution, under the following title : "An Account of a Method of Copying Paintings upon Glass, and of making Profiles by the Agency of Light upon Nitrate of Silver; with Observations by H. Davy." Prom this communication, the following extracts, containing the more important indications, are made.
" White paper, or white leather, 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 changes colour, and after passing through different shades of gray 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 beam of the sun, two or three minutes are sufficient to produce the full effect; in the shade several hours are required; and light transmitted through different coloured glasses acts upon it with different degrees of intensity. Thus, it is found that red rays, or the common sunbeams, passed through red glass, have very little action upon it; yellow and green are more efficacious; but blue and violet light produce the most decided and powerful effects.
" 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 application of water, or water and soap, and it is in a high degree permanent. 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 shade, but in this case the exposure should be only for a few minutes : by the light of candles or lamps, as commonly employed, it is not sensibly affected. No attempts that have been made to prevent the uncoloured parts of the copy or profile from being acted upon by light, have as yet been successful. They have been covered by a thin coating of fine varnish, but this has not destroyed their susceptibility of becoming coloured; and even after repeated washings, sufficient of the active part of the saline matter will adhere to the white parts of the leather or paper to cause them to become dark when exposed to the rays of the sun. Besides the applications of this method of copying that have just been mentioned, there are many others; and it will be useful for making delineations of all such objects as are possessed of a texture partly opaque and partly transparent. The woody fibres of leaves, and the wings of insects, may be pretty accurately represented by means of it; and in this case it is only necessary to cause the direct solar light to pass through them, and to receive the shadows upon leather.
" The images formed by means of a camera obscura have been found to be too faint to produce, in any moderate time, an effect upon the nitrate of silver. To copy these images was the first object of Mr. Wedgwood in his researches on the subject; and for this purpose he first used nitrate of silver, which was mentioned to him by a friend as a substance very sensible to the influence of light; but all his numerous experiments as to their primary end proved unsuccessful. 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 probably 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. (Davy).
" In comparing the effects produced by light upon muriate of silver with those produced upon the nitrate, it seemed evident that the muriate was the most susceptible, and both were more readily acted upon when moist than when dry—a fact long ago known. Even in the twilight, the colour of the moist muriate of silver, spread upon paper, slowly changed from white to faint violet ; though, under similar circumstances, no immediate alteration was produced upon the nitrate.