This second stage of the development, by which a feeble image may be strengthened and rendered more opaque, is sometimes termed "development by precipitation," and should be correctly understood by the practical operator.
Researches of M. Moser.—The papers of M. Ludwig Moser 'On the Formation and Development of Invisible Images,' published in 1842, explain so clearly many remarkable phenomena of occasional occurrence in the Collodion and paper processes, that no apology need be offered for referring to them somewhat at length.
His first proposition may be stated thus:—"If a polished surface has been touched in particular parts by anybody, it acquires the property of precipitating certain vapours on these spots differently to what it does on the other untouched parts." To illustrate this, take a thin plate of metal, having characters excised; warm it gently, and lay it upon the surface of a clean mirror glass for a few minutes: then remove, allow to cool, and breathe upon the glass, when the outlines of the device will be distinctly seen. A plate of polished Silver may be substituted for the glass, and in place of developing the image by the breath, it may be brought out by Mercurial vapour.
The second proposition of M. Moser is as follows:—"Light acts on bodies, and its influence may be tested by vapours that adhere to the substance."—A plate of mirror glass is exposed in the Camera to a bright and intense light; it is then removed and breathed upon, when an image before invisible will be developed, the breath settling most strongly upon the parts where the light has acted. A plate of polished Silver may be used as before instead of glass, the vapour of Mercury or of water being employed to develope the image. An iodized Silver plate is still more sensitive to the influence of the light, and receives a very sharp and perfect impression under the action of the Mercury.
It seems therefore from these experiments and others not quoted, that the surfaces of various bodies are capable of being modified by contact with each other, or by contact with a ray of light, in such a way as to impart an affinity for a vapour; and further, that many of the Salts of Silver are in the list of substances admitting of such modification. But it is also evident that the same condition of surface which causes a vapour to settle in a peculiar manner also affects the behaviour of the Silver Salt when treated with a reducing agent. Thus, if a clean glass plate be touched in certain spots by the warm finger, the impression soon disappears, but is again seen on breathing upon the glass; and if this same plate be coated with a very delicate layer of Iodized Collodion and passed through the Nitrate bath, the solution of Pyrogallic acid will commonly produce a well-defined outline of the figure even before the plate has been exposed to the light. This experiment, although it does not invariably succeed, is nevertheless an instructive one, and shows the necessity of cleaning the plates used in Photography with care. If there be any irregularity in the manner in which the breath settles upon the glass when it is breathed on, a condition of surface exists at that point which will probably so modify the layer of Iodide of Silver, that the action of the developing fluid will be in some way interfered with.
One more remarkable fact observed by M. Moser may be quoted. He finds that the action of light upon the Daguerreotype plate is of an alternating kind: it first gives an affinity for Mercury, and then removes it. "If light acts on Iodide of Silver," he says, "it imparts to it the power of condensing mercurial vapours; but if it acts beyond a certain time, it then diminishes this power and at length takes it away altogether." This is precisely in accordance with phenomena observed also in the Collodion process, where the deposit of metallic Silver is sometime less marked than usual if the plate has been exposed in the Camera beyond the proper period of time.
A curious perversion of the developing process is occasionally met with, in which on the application of the Pyrogallic Acid, the deposit of Silver takes place upon the shadows of the picture, and not upon the lights; hence on viewing the image by transmitted light, the usual appearance is reversed. This may perhaps be explained by an alternating action of the light as above suggested.
A phenomenon at first sight even more remarkable has occurred, in which, on developing the plate, two images start out instead of one. The secondary image in such a case is probably the remains of a previous impression which, although apparently removed by washing, had nevertheless modified the surface of the glass so as to affect the layer of Iodide of Silver; and if the glass were breathed upon before again coating it with Collodion, there is every reason to suppose that the outlines of the accidental image would be seen.[10]
[10] Since writing the above, the Author has perused with pleasure a paper by Mr. Grove on the production of latent images by electricity, with a mode of fixing them. In the experiments described, a plate of glass, electrized in certain portions only, was breathed upon, or exposed to the fumes of Hydrofluoric Acid. In either case the vapour settled exclusively upon the non-electrical part of the glass, thus developing a latent image. When the plate was first submitted to electrization, and then coated with Iodide of Silver upon Collodion, and exposed to light,—solution of Pyrogallic Acid produced a reduction of Silver only upon the parts of the glass corresponding to those on which the breath settled in the previous experiment; thus indicating that the electricity neutralized the effect of light upon the sensitive Iodide of Silver.