PHOTOGRAPHY. Photographic pictures are facsimile representations of objects, pro duced by the chemical action of light on a prepared tablet, upon which the images of the objects are thrown by a camera-obscura. Such a process has been named after its inventor the Daguerreotype ; but other methods have received the names of Calotype, Talbotype, Chrysotype, &e. The process generally may be termed photography (light-drawing), or heliography (sun-drawing). The invention was first formally communicated to the public, by M. Arago, who read an account of the Da guerreotype process before the Academy of Sciences, January 7, 1839. From that mo ment Daguerre (who was afterwards rewarded by a pension by the government) and his in vention engrossed general attention. The dis covery was spoken of as little short of mira culous ; and though it does not realise all the sanguine hopes of its early inventors, the invention is unquestionably highly valuable, because it not only ensures perfect fidelity of likeness where it is most essential, and where it is hardly attainable by the most practised and patient hand and eye, but also gives us the minutest details—those which are imper ceptible to the naked eye, and of course cannot possibly be represented upon paper, yet be come visible in a photographic drawing when it is examined with a magnifying-glass.
In Daguerre's original process, the photo. graphic drawings were produced upon plates of copper coated over with silver. After being washed with a solution of nitric acid, the plate was put into a well-closed box, where it was exposed to the action of iodine, a small quan tity of the latter being placed at the bottom of the box with a thin gauze between it and the plate. A layer of ioduret of silver was thus formed on the surface of the plate, and mani fested itself by the yellow hue produced on the silver, which shows that the process of giving the plate the sensitive coating on which the action of light delineates objects is com pleted. Thus prepared, the plate was next placed within a camera-obscura of particular construction ; and the delineation of the object was then effected in a very short space of time, but had to be afterwards brought out and rendered distinct by another operation, namely, submitting the plate to the action of vapour of mercury. Even then the process was not completed, for the plate had to be plunged into a solution of hyposulpbate of soda, and afterwards washed in distilled water, which being done, the impression was fixed, and the plate could be exposed to light with perfect safety.
Mr. Fox Talbot, and other experimenters, were engaged upon somewhat similar re searches about the same time as M. Daguerre; and within the last twelve years numerous im. provements have been introduced by Herschel, Talbot, Hunt, Claudet, Beard, Kilburn, and others.
By the experiments of these individuals much has been done to render Daguerreotype portraits more pleasing, by improved manage ment of the light, and by placing behind the sitters painted screens, to relieve the head, and to form artificial backgrounds. One of
the greatest difficulties in the original process arose from the circumstance, that, as the image produced in the camera-obscura was totally invisible until brought out by a subsequent exposure to the vapour of mercury, it was im possible to tell precisely at what moment the action of the light should be stopped, to avoid, on the one hand, an image imperfectly de veloped, and, on the other, the misty unde fined appearance occasioned by the unavoid. able motion of the object to be copied (when ever it is an animate object), and the dis coloured or burnt appearance of an image which has been exposed too long to the che mical action of light. This inconvenience has been remedied by many ingenious devices. One important class of improvements has reference to the means of fixing and securing from injury by the subsequent action of light or other means, the Daguerreotype image. Considerable progress has been made towards transferring the colours of natural objects to the Daguerreotypes ; but in the ordinary co loured Daguerreotypes, some of which mako a tolerably near approach to the effect which might be expected if colours could be fixed in the camera obscura, the tinting is produced by the application of finely powdered colours to the surface of the photographic impression, which is previously coated with an alcoholic solution of copal, and nearly dried ; so that the colouring, which is so delicately performed as not to impair the distinctness of the im pression, is not in any degree the result of the action of light.
Most of the numerous patented or proposed improvements in the art depend chiefly on the mode of producing the pictures. The Chro matype, Chrysotype, Calotype, Cyanotype, Am phitype, and Ferrotype, all are so many modes of producing and fixing the impressions. Some of the improvements, however, relate to the mechanism of the camera-obscura, in which the sensitive surface is placed. See further on this point under AMPILITYPE ; CALOTYPE ; CHROMATYPE.
Mr. Brunel has lately registered an appa ratus called a Photographotrope, comprising all the necessary materials for taking and fixing a photographic picture, packed up so as to stand upon a tripod or simple support. There is at the top of the apparatus a camera for producing the focal image of the object ; and there are the proper means for impressing this image upon a metal plate. Below this is a spirit lamp in a tube, and a recess in which the plate is to be placed to be acted on by heated mercurial vapour. Below this again, is a small galvanic battery, which expedites the photographic process.