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 day light, 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 beams 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 apon 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. [footnote 1]
The consideration of these facts enables us readily to understand the method by which the outlines and shades of paintings on glass may be copied, or profiles of figures procured, by the agency of light. When a white surface, covered with solution of nitrate of si1ver, is placed behind a painting on glass exposed to the solarlight; the rays transmitted through the differently painted surfaces produce distinct tints of brown or black, sensibly differing in intensity according to the shades of the picture, and where the light is unaltered, the colour of the nitrate becomes deepest.
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 upon than when paper is used.
After the colour has been once fixed upon 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 with 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 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.
Besides the applications of this method of copying that have been just 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 prepared leather.
When the solar rays are passed through a print and thrown upon prepared paper, the unshielded parts are slowly copied; but the lights transmitted by the shaded parts, are seldom so definite as to form a distinct resemblance of them by producing different intensities of colour. 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 the nitrate of silver, which was mentioned to him by a friend, as a substance very sensitive 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 sular 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.
With regard to the preparation of the solution, I have found the best proportions those of 1 part of nitrate to about 10 of water. In this case, the quantity of the salt applied to the leather or paper, will be sufficient to enable it to become tinged, without affecting its composition, or injuring its texture.
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 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.
The nitrate, however, from its solubility in water, possesses an advantage over the muriate: though leather or paper may, without much difficulty, be impregnated with this last substance, either by diffusing it through water, and applying it in this form, or by immersing paper moistened with the solution of the nitrate in very diluted muriatic acid.
To those persons not acquainted with the properties of the salts containing oxide of silver, it may be useful to state, that they produce a stain of some permanence, even when momentarily applied to the skin, and in employing them for moistening paper or leather, it is necessary to use a pencil of hair, or a brush.
From the impossibility of removing by washing, the colouring matter of the salts from the parts of the surface of the copy, which have not been exposed to light; it is probable, that both in the case of the nitrate and muriate of silver, a portion of the metallic oxide abandons its acid, to enter into union with the animal or vegetable substance, so as to form with it an insoluble compound. And, supposing that this happens, it is not improbable, but that substances may be found capable of destroying this compound, either by simple or complicated affinities. Some experiments on this subject have been imagined, and an account of the results of them may possibly appear in a future number of the Journals. Nothing but a method of preventing the unshaded parts of the delineation from being coloured by exposure to the day is wanting, to render the process as useful as it is elegant.
The facts above mentioned are analogous to those observed long ago by Scheele, and confirmed by Senebier. Scheele found, that in the prismatic spectrum, the effect produced by the red rays upon muriate of silver was very faint, and scarcely to be perceived; whilst it was speedily blackened by the violet rays. Senebier states, that the time required to darken muriate of silver by the red rays, is 2.0 minutes, by the orange 12, by the yellow 5 minutes and 30 seconds, by the green 37 seconds, by the blue 29 seconds, and by the violet only 15 seconds. Senebier sur la Lumiere, Vol. II. p. 199.
Some new experiments have been lately made in relation to this subject, in consequence of the discoveries of Dr. Herschel concerning the invisible heat-making rays existing in the solar beams, by Messrs. Ritcer and Bockmann in Germany, and Dr. Wollaston in England.
It has been ascertained, by experiments upon the prismatic spectrum, that no effects are produced upon the muriate of silver by the invisible heat making rays which exist on the red side, and which are least refrangible, though it is powerfully and distinctly affected in a space beyond the violet rays out of the visible boundary.
See Annalen der Physik, siebenter Band, 527.