Natural Colored Photographs.

Scientific American 12, 21.3.1863

The celebrated French chemist, Mr. Niepce De Saint Victor, has been for many years devoting himself to experimental heliochromy, for tho purpose of discovering the art of taking fixed photographs in their natural colors. He has lately presented his fifth memoir on the subject to the Academy of Sciences, Paris. The following are some extracts from it:-

"I have always found yellow the color most difficult to obtain in the same space of time as the other tints; but I have recently discovered the means of developing it with certainty, and of obtaining it in the same time as other colors. I had previously obtained, with great facility, red, green, and blue; I have arrived at obtaining yellow, by employing as an agent for chloridizing my plates, a bath composed of hypochlorite of soda, in preference to the hypochlorite of potent. This bath must be in the following conditions:

— Take newly-prepared hypochlorite of soda, marking six degrees of the areometer; dilute it with onehalf its bulk of water, and then add alcohol in quantity equal to ½ per cent of the soda, and heat the bath to a temperature of 180° to 190°, Fah.; then pour it into a flat capsule, half-plate size, stirring the liquid for a few seconds, immerse the plate in it at once, a time sufficient for the plate to take a black tint. It Is then rinsed in abundance of water, and dried over a spiritlamp. In 200 grammes (6¼ oz) of this bath we can chloridize five or six quarter-plates, among which some will give better results than others, according to the thickness of the film and the degree to which the plate has been heated. In these conditions of chloridization the colors are produced (especially by contact) with very vivid tints, and very frequently the blacks appear in their full intensity. To operate in the camera obscure, we select plates which, by the action of heat, have received a fine cherryred tint, as well as those which are more slightly reheated, because they are the most sensitive to light. On this account the film of chloride of silver must not be too thick. Rot, to obtain the effects which I now describe, the chlorldized plate must be covered with a varnish with a base of chloride of lead.

"With regard to the problem of fixing the colors, I have only suoceeded in doubling the time of duration announced in my last report. Many substances, added after the action of heat upon the chloride of lead, give a greater fixity than if the chloride of lead was alone; such are, among others, the tincture of benzoic, chloride of tin, and aldehyde. Rut what has given me the best result is the tincture of Siamese benzoin, applied to the plate while it is yet warm, and, after the plate has become dry, heating it until a little of the benzoic acid is volatilized. It Is by means of this lead varnish that I have been enabled to preserve colors during three er four days, in an apartment strongly illuminated by daylight, in the month of July. If we incline a heliochromic Image, at a certain degree of incidence, the color, appear much more vivid, and the blacks assume the greatest intensity. I have also remarked that, according to the manner in which the figure of a doll (which I used) is illuminated by the solar rays, the obtaining the colors in the camera obscure becomes singularly modified, and produces very advantageous effects as to intensity of color and brilliancy; as, for example, gold and silver lace, precious stones, &c. But what is very extraordinary, is that, having placed a strip of unglazed black paper upon a large piece of silver lace, which the figure wore as a belt, the black of the paper was reproduced with the white of the silver lace. Black is reproduced with a violet hue, viewed direct; but, if the plate be inclined at a certain angle, it assumes its greatest intensity, and the silver lace its metallic splendor. Light, in changing tho holiochromic colors made in certain cases, changes green into blue, and yellow into green; as, for instance, if we cover them with a varnish having chloride of tin for a base, which, moreover, greatly retards the activity of the light; if it had not this objection, it would serve as a temporary fixing agent, for the reds are preserved a very long time.

"I have proved that all the binary colors are decomposed by heliochromy. If the green be natural, like that of the emerald, arsenite of copper, oxide of chrome, sulphate of nickel, green carbonate of copper, they are reproduced green by heliochromy; but if the green be a compound, like that, for example, formed by a mixture of Prussian blue and chrome yellow, or that of stuffs dyed by means of a blue coloring material and a yellow, or of certain glasses colored by blue and yellow pigments, these greens, I repeat, give blue only by heliochromy, either by contact, or in the camera obscure.

A light blue glass, superimposed upon a light green glass, give by transparency, a very fine green; but, being applied to a heliochromic plate, they only produce blue; whatever be the time of exposure to the light, or whether the blue glass be uppermost or below, the results are the same. Certain kinds of green glen reproduce green very well; others give only blue or yellow effects. There are also other examples: a red glass, superimposed upon a yellow glass, giving an orange by transparency, produces only red upon the sensitive plate. A red glass, superimposed upon a blue glass, giving violet by transparency, first produces a violet, then blue follows: the red being replaced by an orange green, also quickly reproduces blue. A white paper, colored green by green leaves, is reproduced only very slowly by contact: the sensitive plate remains red a very long time, as if the light had no action; and if the exposure be prolonged, a bluish grey tint in produced; the same result takes place if we attempt to reproduce natural foliage in the camera, such as, for instance, the herbage of a green meadow; but if the foliage be a bluegreen, as, for instance, the leaves of the dahlia, the blue tint will be more vivid. If the foliage be yellow or red, like that of dead leaves, the color reproduced will be a yellow or a red, more or less pure, accord. ing to the greater or lesser absence of the blue matter, which, with the yellow, constitutes the green color of leaves. The dye of a peacock's feather is well reproduced in the camera, that is, the color appears under a certain degree of incidence, now green, now blue."

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