Puslapio vaizdai

are both definite componnds, is of course at present merely my conjecture; that they are signally different, is evident from their dissimilar properties.' *

Both Sir John Herschel and Mr Hunt concur in the theory given of these spots by Mr Talbot; and the former has suggested the following method of preventing their occurrence.

• It frequently happens that, however carefully the successive washes are applied, so as apparently to drench completely every part of the paper, irregular patches in the resulting sheet will be of a comparatively much lower degree of sensibility; which degree is nevertheless uniform over their whole area. These patches are always sharply defined and terminated by rounded outlines, indicating, as their proximate canse, the spreading of the wash last applied within the pores of the paper. They have been noticed and well described by Mr Talbot, and ascribed by him, I think justly, to the assumption of definite and different chemical states of the silver within and without their area, which would be highly in. teresting to follow out. They are very troublesome in practice, but may be materially diminished in frequency, if not avoided altogether, by saturating the saline washes used, previous to their application, with chloride of silver. By attending to this precaution, and by dividing the last wash of the nitrate into two of half the strength, applied one after the other, drying the paper between them, their occurrence may be almost entirely obviated.

The occurrence of these white spots on the paper used for positive photographs, is particularly distressing. When a favourabl sun and a fine negative drawing should have produced a powerful picture, the figures often appear without heads or hands, or with such numbers of white spots as to destroy the picture. In order to be secure against this disappointment, Sir David Brewster exposes the nitrated paper to such a degree of light as to produce a sort of neutral brownish tint over the whole. The uniformity of this tint indicates the absence of white spots; and when the white spots do appear, we may either reject the paper or place the negative upon that part of it which is uniformly tinged. This tinge has another advantage. It prevents that disagreeable change of colour, which, in the course of time, comes over all photographs that have been fixed with the bromide of potassium; and it greatly adds to the effect of a picture with very deep shadows produced by an excess of light, and which has been fixed by ammonia.

Within our present limits, we cannot stop to give our reader

* Some account of the Art of Photogenic Drawing, &c.'p. 13.

| Instead of using for positives the strong nitrate of 80 grains to 1 oz. of water, he uses the aceto-nitrate, with only 50 grains to l} oz. of fluid, that is, of water and acetic acid. The acetic acid may be replaced by common vinegar in taking positives.

[ocr errors][ocr errors][merged small]

an i

of yo
of the

Som. round, Very it regard

at presa

The pro

compou the two clreumatu

In pursuing these researches, this distinguished philosopher has been led to other highly interesting results. The action of light on vegetable colours, he finds to be positive ; that is, it either destroys the colour totally, or leaves a residual tint on which light has no further action.* This action is confined to the region of the spectrum occupied by the luminous rays; and the rays which are effective in destroying any given tint, are in a great many cases

those whose union produces a colour complementary to the tint • destroyed.' A still more interesting result of this enquiry has been the discovery of two new Photographic processes ; to the latter of which its author has given the name of Chrysotype, from its being chiefly produced by a solution of gold. When paper has been first washed over with a solution of ammonio-citrate of iron, then dried, and afterwards washed over with a solution of ferrosesqui-cyanuret of potassium, it becomes capable of receiving with great rapidity a positive photographic impression.

When a negative picture has been impressed upon paper washed with the former of these solutions, but which is originally faint and sometimes scarcely perceptible, it is immediately called forth upon being washed over with a neutral solution of Gold. The picture does not at once acquire its full intensity, but rapidly blackens up to a certain point; when the photograph acquires a sharpness and perfection of detail which nothing can surpass. A solution of silver produces a similar effect with greater intensity, but much more slowly.t

To Professor Draper of New York, we owe many interesting facts and views connected with the photographical art. He was the first, we believe, who, under the brilliant summer sun of New York, took portraits with the Dagnerreotype. This branch of Photography seems not to have been regarded as a possible application of Daguerre's invention; and no notice is taken of it in the reports made to the legislative bodies of France. We have been told that Daguerre had not at that period taken any portraits; and when we consider the period of time, twenty or twenty-five minutes, which was then deemed necessary to get a Daguerreotype landscape, we do not wonder at the observation of a French author, who describes the taking of portraits as toujours un terrain un peu fabuleux pour le Daguerreotype. Daguerre, however, and his countryman, M. Claudet, have

* This effect is perfectly analogous to that of the action of heat upon the colour of minerals. In Brazil topaz the residual tint is always a light pink. See Phil. Trans. vol. xix. p. 25.

† Hence Sir J. Herschel considers the name Siderotype, taken from the iron employed in one of the solutions, as preferable to Chry'

nobly earned the reputation of having perfected this branch of the art.

It had been long known, that if we write upon a piece of glass with a pencil of Soapstone or Agalmatolite, the written let. ters, though wholly invisible, may be read by simply breathing upon the glass; and this even though the surface has been well cleaned after the letters had been written. Dr Draper observed, that if a piece of metal, a shilling for example, or even a wafer, is laid upon a cool surface of glass or polished metal, and the glass or metal breathed upon, then, if the shilling is tossed from the surface, and the vapour dried up spontaneously, a spectral image of the shilling will be seen by breathing again upon the surface; the vapour depositing itself in a different manner upon the part previously protected by the shilling.* More recently, Protessor Draper has shown, that this spectral image could be revived during a period of several months of the cold weather in the winter of 1840-1; but he has stated that he cannot find the reason of this result, though he regards it as analogous to the deposition of mercurial vapour in the Daguerreotype. We have often repeated this interesting experiment, by keeping the protecting body, the shilling or wafer, at a distance from the glass or metallic surface, or by putting it under a watch-glass; and we found that the result was always the same, (even after cleaning the surface with soft leather, so that change of temperature, or any pressure upon the glass surface, were excluded as causes of the phenomenon.

Professor Draper was led also to the interesting conclusion, • that the chemical action produced by the rays of light, depends

upon the rays being rendered latent or absorbed by sensitive bo• dies ;' that by some unknown process, photographic effects on * sensitive surfaces gradually disappear, and that it depends on • the chemical nature of the sensitive material, which rays shall be rendered latent or absorbed.'!

During a long journey, undertaken during the last summer for the purpose of trying the photographical power of the sun's rays in lower latitudes, Professor Draper has been conducted to a very remarkable discovery. No similar result could be obtained at New York, and therefore we can have no expectation of witnessing it in England. From photographic impressions of the solar spectrum, obtained in the South of Virginia, when the thermometer was 96° of Fahrenheit in the shade, Professor Draper found, that ' under a brilliant sun, there is a class of rays com

[ocr errors]
[ocr errors]

* Lond. and Edin. Phil. Magazine, vol. xviii. p. 218, Sept. 1840-41, † Id. Id. v, xix. 198.

| Id. Id. 195-6,


mencing precisely at the termination of the blue, and extending • beyond the extreme red, which totally and perfectly arrest the • light of the sky. The negative rays seemed almost as effective • in protecting, as the blue rays are in decomposing iodide of • silver.'

The most remarkable part of the phenomenon,' says Professor Draper, • is, that the same class of rays makes its appearance again beyond the extreme lavender rays. Sir J. Herschel has already stated, in the case of bromide of silver, that these negative rays exist low down in the spectrum. This specimen, however, proves that they exist at both ends, and do not at all depend on the refrangibility. It was obtained with yellow jodide of silver, Daguerre's preparation, the time of exposure to the sun fifteen minutes.

• In this impression, six different kinds of action may be distinctly traced, by the different effects produced on the mercurial amalgam. Those, commencing with the most refrangible rays, may be enumerated as follows:-Ist, protecting rays; 2d, rays that whiten ; 3d, rays that blacken; 4th, rays that whiten intensely; 5th, rays that whiten very feebly ; 6th, protecting rays.

• It is obvious we could obtain negative photographs by the Daguerreotype process, by absorbing all the rays coming from natural objects, except the red, orange, yellow, and green, allowing at the same time diffused daylight to act on the plate.

• This constitutes a great improvement in the art of Photography, because it permits its application in a negative way to landscapes. In the original French plan, the most luminous rays are those that have least effect, whilst the sombre blue and violet rays produce all the action. Pictures produced in that way never can imitate the order of light and shadow in a coloured landscape.”

From these observations, Professor Draper considers that • there are strong reasons for believing that the sun's light, in

tropical seasons, differs intrinsically from ours.' With a French achromatic lens, which performed admirably in a camera at New York, the Chevalier Fredrichstal, who travelled in Central America for the Prussian government, found very long exposures in the camera necessary, to produce impressions of the ruined monuments of the deserted cities. Professor Draper says that these Daguerreotypes are of a very remarkable aspect ; and he • assures us that other competent travellers experienced similar • difficulties, and even failed to get any impressions whatever.' These difficulties must certainly be due, as Professor Draper conjectures, to the antagonist action of the negative and posi

" *

tive rays.

We shall now give our readers a very condensed account of the extraordinary discoveries recently made by M. Ludwig Moser,

* Lond. and Edin. Phil. Magazine, vol. xxi. p. 319. VOL. LXXVI. NO, CLIV.

[ocr errors]
« AnkstesnisTęsti »