Great Facts / A Popular History and Description of the Most Remarkable Inventions During the Present Century

by Frederick C. Bakewell

[PHOTOGRAPHY.]

The power we now possess of fixing the transient impression of the rays of light, and of retaining the beautiful images of the camera obscura, is perhaps the most astonishing of the present age of wonders. Effects similar to those of the electric telegraph, of steam navigation, of dissolving views, and of other wondrous realizations of inventive genius, had been anticipated in growing tales of Eastern romance centuries ago; but the most fanciful imagination had not conceived the possibility of making Nature her own artist, and of producing, in the twinkling of an eye, a permanent representation of all the objects comprehended within the range of vision.

Such an idea could scarcely have occurred until after the invention of the camera obscura; but when looking at the beautiful pictures focused on the screen of that instrument, it became an object of longing desire to fix them there.

To trace the history of Photography from its earliest beginnings, we must go back to the days of the alchemists, who were the discoverers of the influence of light in darkening the salts of silver, on which all photographic processes on paper depend. That property of light was noticed in 1566, and it induced the speculative philosophers of that day to conceive that luminous rays contained a sulphurous principle which transmitted the forms of matter. Homberg, more than a century afterwards, misled by this action of the sun's rays, supposed that they insinuated themselves into the particles of bodies, and increased their weight; and Sir Isaac Newton also entertained a similar opinion.

The influence of the solar rays in facilitating the crystallization of saltpetre and sal ammoniac, was shown by Petit in 1722; and in 1777, the distinguished chemist Scheele discovered that the violet rays of the spectrum possess greater power in producing those changes than any other. A solution of nitrate of silver, then called "the acid of silver," was known to be peculiarly susceptible to the action of those rays. The experiment by which it was illustrated consisted in pouring the solution on chalk, which became blackened by exposure to light. These discoveries were made by Scheele in his endeavours to find in light the source of "phlogiston"—that ignis fatuus of the chemists of the last century. We thus perceive, in the first steps towards the invention of Photography, one of the many instances of the discovery of truth in the search after error.

At the beginning of the present century, Mr. Wedgwood, the celebrated porcelain manufacturer, undertook a series of experiments to fix the images of the camera, assisted by Mr. (afterwards Sir Humphry) Davy. They so far succeeded as to impress the images on the screen, but unfortunately they had not the power of preserving the paper from being blackened all over when exposed for a short time to the light. "Nothing," said Sir Humphry Davy, in his account of these experiments, "but a method of preventing the unshaded parts of the delineation from being coloured by exposure to light is wanting to render this process as useful as it is elegant."

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.