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himself in dyed garments—it must have been observed that some colours were darkened, while others were bleached by the sun's rays. To the philosophy of this, his mental eye was obscured—the fact was constantly occurring (and a thousand facts are still for ever presenting themselves to us, unnoticed or uncared for), and man did not perceive the important bearing of the phenomenon.

Eventually, the alchemists, possessed with the idea that gold differed from silver in nothing but that it contained more of the sun's sulphur, were induced to present various compounds of silver to the sunshine, with the hope of obtaining this “ interpenetration of the sulphureous principle of light, which was to change the baser silver to the royal gold.

Eventually an Englishman, Mr Thomas Wedgwoodthe son of him who so greatly improved our porcelain manufacture-conceived it quite possible, since differentcoloured media were not equally transparent to the radiant chemical power, to copy the paintings on the windows of our old churches by covering white paper or leather with the nitrate or the chloride of silver. He succeeded in his experiments, and, with the assistance of Sir Humphry Davy, extended his plan so far as to secure copies of images by the solar microscope, thus becoming the discoverer of the beautiful art of PHOTOGRAPHY. The pictures produced by Mr Wedgwood wanted permanence. They could only be preserved in the dark. Viewed by daylight they soon became uniformly black. A few years after this a French gentleman, M. Niepce, was induced to take up the inquiry, and he made the remarkable discovery, that the solar rays altered the character of all kinds of

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resinous substances. He therefore spread upon plates of glass and metal a thin coating of some varnish, and placing it in the camera-obscura, allowed the beautiful images of Baptista Porta's instrument to fall upon the plate. *

These images, being the result of radiations from external objects, have relatively the amount of luminous and chemical power determined by the colours of their surfaces, and the quantity of illumination to which they are exposed. It was found, after exposure in this way, that some portions of the resinous surface were more soluble than others. The plates were consequently placed in some solvent, and thus was gradually developed "the clouded imagery" of the picture impressed upon the plate. Many of the pictures thus produced ---called by their discoverer HELIOGRAPHST-are still to be found in this country, M. Niepce, being involved in the revolutionary troubles of France, having sought safety and repose at Kew. Niepce eventually returned to Paris, and then became acquainted with Daguerre, the dioramic painter. They were both engaged in the same line of inquiry, and it was agreed that they should continue their investigations together. It is not quite easy to trace the progress made by Niepce and Daguerre, as it was not until after the death of Niepce that Daguerre announced the discovery of the process which bears his name.

During this period Mr Henry Fox Talbot was quietly * The Camera-obscura is too well known to require a description. It was the discovery of an Italian philosopher, Baptista Porta, in the sixteenth century.

+ Meaning Sun-drawing—a name far more happily chosen than PHOTOGRAPHY, or Light-drawing, modern science having shewn that the chemical changes are not due to the light-producing power of the sunbeam, but to an associated dark principle called ACTINISM.

working in the same direction, and he so far improved upon the process of Wedgwood as to give permanence to the sun-drawn pictures. Since the publication of these processes, photography has made rapid advances.

A few of the more important processes must now be described. It is difficult, within the limits allowed, to make a selection from, or to enter into the details of, the various methods by which photographs can be obtained ; the most satisfactory course will be to state those general principles by which the resulting photographic phenomena may be best understood.

If silver is dissolved in nitric acid, we obtain a saltnitrate of silver. When this salt is dissolved in perfectly pure distilled water, it may be exposed to sunshine for any period without undergoing change ; but add thereto the smallest portion of organic matter, and it is quickly decomposed, the silver being precipitated as a black powder. In paper we have the required organic principle, and if we wash a sheet with the solution of nitrate of silver, and expose it with any body superposed-say a fern leaf—all the parts which are exposed will blacken, those screened will remain white, and thus there will be produced what is called a negative image. Chloride of silver, obtained by washing the paper, first with a weak solution of common salt and then with nitrate of silver, is a far more sensitive photographic agent, and is now commonly employed.

The Calotype process of Mr Fox Talbot consists in washing paper first with iodide of potassium, and then with nitrate of silver, by which process is obtained an iodide of silver. The paper should contain nothing but this iodide; therefore all soluble salts are removed by soaking in water. This pale primrose-colour paper, which is not sensitive to light, is washed with a peculiar organic salt called gallic acid; and to increase the instability of the preparation, a little nitrate of silver is added to it, producing what the inventor calls a gallo-nitrate of silver. Here we have a preparation already quick with chemical energy; this is applied to the iodised paper, and the chemical power of the sun, as radiated from external objects, instantly produces a change—that change bearing an exact relation to the intensity of the rays falling upon each portion of the light-created picture.

Presently a picture becomes visible, and it is increased in intensity by washing it, in the dark, with a fresh portion of the gallic acid solution. The picture thus obtained is fixed by washing it with a salt, which dissolves the iodide or the chloride of silver, which has not undergone change—the hyposulphite of soda—and subsequently soaking in clean water.

The Daguerreotype consists in producing an iodide of silver upon the surface of a polished silver plate, and receiving the camera image upon this prepared surface. In both of these processes a decomposition of the iodide of silver results; but in Daguerre's process, the image is developed by exposing the plate on which it has been impressed to the vapour of mercury.

Mercury combines with metallic silver, but not with the iodide; thus it is deposited over every portion of the plate on which the solar radiations have acted—the thickness of the deposit bearing a strict relation to the intensity of chemical effect produced. This picture is also fixed by the

use of the hyposulphite of soda; as, indeed, are nearly all varieties of photographic pictures.

By modifications, which cannot be here detailed, these processes have been greatly increased in sensibility; the result which formerly required twenty minutes being now obtained in as many seconds.

A process more sensitive than either of those named has extended photography in a most remarkable mannerthis is the COLLODION process. Collodion is gun-cotton dissolved in ether; to this is added some iodide of potassium dissolved in spirits of wine. This iodised collodion is poured over a sheet of glass—the ether evaporating leaves a beautiful film on the surface, which, upon the glass being dipped into a solution of nitrate of silver, becomes exquisitely sensitive. This prepared tablet being placed in the camera receives an image almost instantaneously, which is brought out in full vigour by pouring over it a solution of the proto-sulphate of iron or of pyrogallic acid.

The exquisite perfection of the collodion pictures, dependent upon the rapidity with which the images are impressed, is mainly due to the peculiar conditions of this singular preparation. By a preparation in many respects analogous to the collodion, a degree of sensibility far exceeding anything which the most sanguine photographist dreamed of in his ardent moments has been obtained.A plate prepared with albumen, iodide of iron, and alcohol, and acetic acid, was placed in a dark room of the Royal Institution in a camera obscura ; opposite to it, at the proper focal distance, was a wheel, which was made to revolve many hundred times in a second, and this wheel

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