van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.
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Illustrated with many Woodcuts.
Virtue Brothers & CO., 1, Amen Corner,
THE high repute of D. Van Monckhoven on the Continent as a practical photographer, and the very favourable reviews which his works have from time to time received from the leading photographic journals in this country, have led to the following free translation of his "Traité Populaire de Photography sur Collodion." The alterations and emendations which have been made in the original text, it is hoped, will render the whole more immediately available to the English reader, and acceptable as a handbook of photographic art.
AMONGST the various methods which have been from time to time proposed for the production of pictures by the chemical agency of light, and comprehended under the general term Photography, there are only four--the DAGUERREOTYPE, CALOTYPE, ALBUMEN, and COLLODION processes--which have been to any extent practically successful.
The photographic picture is obtained in the Daguerreotype process, on polished metal plates; Chloride, on paper; Albumen, on a film of albumen on glass; Collodion, on a film of Collodion on glass.
And it is an interesting fact that, although at first sight these processes do not appear to have any connection with each other, there nevertheless does exist one general principle of action, which will be obvious, even to the superficial observer.
THE DAGUERREOTYPE.--This process was discovered by Niepce and Daguerre. A silver, or silvered copper, plate, highly polished, is placed in the dark on a china dish, containing iodine. The vapour of this substance combines with the silver of the metallic plate in such a manner as to produce iodide of silver, a substance sensitive to light. In fact, it is sufficient to expose it behind a perforated card in the daylight for some considerable time to produce an impression; but should the time of exposure be much shortened there will not be any picture visible on the layer of iodide of sliver, although such really exists, and can be developer, or made to appear. This can be effected by placing, the plate over mercury heated to about 148° Fahr., so that it receives the vapours which arise there from, when an exact image of the perforated card will be apparent in a very few seconds. There always remains a certain quantity of iodide of silver which has not been altered by light, because it has been protected from the action of this agent by tile opaque body which covered the plate in certain places. If this iodide of silver were not removed, it is easy to understand that it would become changed as soon as exposed to the light; and therefore it is necessary to dissolve it by a body which has received the name of a fixing agent. Many substances dissolve iodide of silver, such as the various iodides, cyanides, and alkaline hyposulphites; but amongst these the most useful is the hyposulphite of soda.
The Daguerreotype process just described will be perceived to essentially consist of a layer of iodide of silver, the use of a developer to bring out the latent picture, and a fixing agent for removing that portion of the iodide of silver not acted upon by the light.
THE CALOTYPE process was the invention of Mr. Fox: Talbot, and, in consequence, is sometimes termed Talbotype. It consists in spreading, on a sheet of paper, first a solution of iodide of potassium, and then a solution of nitrate of silver. These two bodies, by their mutual reaction, produce a yellowish-white, insoluble powder; then, by washing the paper in water contained in a porcelain basin; all the excess of nitrate of silver is carried away, and finally there results a paper the pores of which are completely impregnated with iodide of silver. If this for sect' paper be exposed to the light, behind a perforated card, the iodide of silver will become black at the places where the light strikes upon it; or if a very short exposure be given to the paper, on examination by yellow light no image will be perceived. The picture can, however, be developed by immersing tile paper in an aqueous solution of gallic acid, mixed with a very small quantity of another aqueous solution of nitrate of silver and acetic acid. This mixture constitutes Mr. Talbot's developer,
At the end of some minutes the image shows itself, increases in vigour, and becomes very distinct; the paper is then removed from the basin of gallo nitrate of silver, washed in water, and the iodide of silver not affected by the light is dissolved out with a solution of hyposulphite of soda.
The two processes of Daguerreotype and calotype, although employing such different substances as metal and paper, nevertheless have many points of resemblance, as in both the pictures are, obtained on iodide of silver, requiring development and subsequent fixing; but when the respective pictures are examined, a very important difference will be perceived in their general aspect. A proof taken by the Daguerreotype will exhibit a counterpart of the original, with all the lights and shades correct. If it be a view taken with the aid of a camera, the sky will appear white, the trees a little darker, and the shadows black as in nature; and, in the case of a portrait of a person standing before a white wall, the picture of the wall will be white, tile hair and features differently tinted, and the dress black; consequently, it is usual to call this image direct, or positive.
It is quite different with the Calotype. picture; in fact, the view will have a black sky, the trees lighter, and the shadows white; and, if it were the portrait, the wall black and the dress white; or, in other words, the picture will in every respect, as to depth of tint, be the reverse of the original, and for this reason is called indirect or negative.
Figs. 1 and 2 give an idea of these negative and positive images. Thus Fig. 1 represents a black cross on a white ground. If it be reproduced by the Daguerreotype an exact copy is obtained; whilst if the calotype process be used the result will be an inverted image, or the cross will be white on a black ground; in fact a negative image, as shown at Fig. 2. Figs. 3 and 4 show another example of these negative and positive proofs.*
There are many drawbacks against the employment of the Daguerreotype process; for instance, the plate possesses a dazzling brightness, which forces the observer to incline himself in some favourable angle to be able to examine the details of the picture; besides, it must be protected by glass, because the image which it bears on its surface is destroyed by the least friction; lastly, and this is above all its principal disadvantage, it only gives a single image, whilst the other methods furnish an indefinite number. When a negative image is once obtained, it can be employed to produce a series of other images, which will also be reversed in relation to the negative, and consequently positives. Take for example Figs. 3 and 4.
Suppose Fig. 3 the model to be reproduced, Fig. 4 will be the negative image on the paper. But if the latter be laid on another sensitive sheet of paper, and exposed thus arranged to the sun, its rays will pass through the white parts, and impress the sensitive paper which is underneath, but will be stopped by the black parts; and thus will the facsimile of Fig. 3 be produced. It will be understood that the same negative Fig. 4 can be used as often as required, and an unlimited number of positives analogous to Fig. 3 thus obtained.
From the necessity and manner of employing a paper negative to produce the required positive impressions, a very correct idea will be formed of the motives which have led photographers to replace paper by a more homogeneous substance; however fine a surface paper may appear to possess, if it be examined by transmitted light it is always very uneven in texture, which circumstance greatly injures the delicate tints and sharpness of detail in the positive proofs.
THE ALBUMEN PROCESS.--The white of an egg, otherwise called albumen, is a transparent liquid, which, spread on a plate of glass, leaves by evaporation a coating as clear as the glass itself, so that when employed as a photographic vehicle the roost minute details are preserved with perfect fidelity.
The manner of operating with albumen is exactly the same as for the negative paper. In the albumen, properly prepared, is dissolved a small. quantity of iodide of potassium; this is spread on a well cleaned plate of glass; the dried glass is immersed in nitrate of silver, exposed to the light in a camera, developed as a proof on paper, and fixed by hyposulphite of soda.
The advantage that albumen offers over paper, is the production of finer details in the picture. It is not, however, at all an easy or sensitive process, and therefore unsuitable for taking portraits, as at least ten minutes exposure is required, even in. a very good light, to produce an impression.
THE COLLODION PROCESS, which employs a film of collodion spread on glass, possesses all the good qualities of albumen, with the very great advantage of being, at least, sixty times more sensitive, and withal easier of execution. It is this process, and the subjects immediately connected therewith, that will be treated upon in the following chapters.