It was in June, 1802, that Mr. T. Wedgwood published "an account of a method of copying paintings on glass, and of making profiles by the agency of light; with observations by H. Davy." Mr. Wedgwood made use of white paper or white leather, moistened with a solution of nitrate of silver. The following description of the process, contributed to the "Journals of the Royal Institution" by Davy, will be read with interest, as showing how closely these experiments approximated to the photogenic process, invented by Mr. Talbot thirty-six years afterwards:--
"White paper or white leather moistened with a solution of nitrate of silver undergoes no change in a dark place; but on being exposed to daylight, it speedily changes colour, and after passing 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 rays 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 on 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 on it. Yellow or green are more efficacious; but blue and violet light produce the most decided and powerful effects.
"When the shadow of any figure is thrown on the prepared surfaced, 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. When the colour has been once fixed on 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 a profile, immediately after being taken, must be kept in a dark place. It may, indeed, be examined in the shade, but in this case the exposure should only be for a few minutes; by the light of candles or lamps, 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 with a 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 still adhere to the white parts of the leather or paper, to cause them to become dark when exposed to the rays of the sun.