DETAILS OF THE TALBOTYPE PROCESS. 887 Each sheet of paper is then to be hung up separately and allowed to dry. These operations may be conducted by candle light or in diffused daylight. A stock of this paper may be prepared and kept for use. 2. Exciting the Paper for the Camera.-When required for the camera, prepare 1. a solution of aceto-nitrate of silver (consisting of 50 grains of nitrate of silver, 1 ounce of water, and 1 drachm of glacial acetic acid), and 2. an aqueous solution of gallic acid saturated in the cold. Add 3 or 4 drops of each of these solutions to I drachm of distilled water, and then in a darkened room apply the mixture freely with a pledget of clean cotton wool to the silvered surface of the iodized paper-when well soaked remove the superfluous portion with a sheet of clean blotting-paper: the same sheet of blotting-paper must not be used twice for this purpose. Whilst still damp it is to be placed between the glasses of the camera slide. It will retain its whiteness for twelve hours or more. 3. Exposure in the Camera.-In order to take a landscape, a sheet of the prepared paper is exposed in the focus of the camera, and after the lapse of from five to fifteen or twenty minutes, according to the amount of light, the picture may be withdrawn. 4. Development.-The image is developed by brushing the paper over, by means of clean cotton wool, with a mixture of equal parts of the solution of aceto-nitrate of silver and gallic acid. The two solutions must be mixed immediately before they are used, as they speedily undergo mutual decomposition. In a few minutes. the picture gradually begins to appear. Any part of the picture which seems wanting in distinctness may be washed over with fresh solution of aceto-nitrate of silver. The development should be effected by candlelight, or in yellow light. 5. Fixing the Impression.-As soon as the picture ceases to acquire distinctness, it is to be well washed with water, and immersed in a saturated solution of hyposulphite of sodium till the yellow tint of the iodide of silver has disappeared. It is then to be washed thoroughly for several hours in clean water, frequently renewing the water. Unless all traces of the hyposulphite of silver be removed, the picture will gradually lose its intensity. Fox Talbot originally employed a solution of bromide of potassium for fixing these pictures, but the hyposulphite of sodium is to be preferred. When dry, the photograph should be waxed by placing it between two sheets of blotting-paper saturated with white wax, and then passing a moderately heated smoothing iron over the whole. The negative pictures thus obtained may be employed to furnish 888 PHOTOGRAPHY ON COLLODION. positive prints, or prints with the lights and shadows as they occur in nature, by Talbot's original photogenic' process, or by printing upon a second sheet of the prepared Talbotype paper.* (1018) Photography on Collodion.-An important modification of Talbot's process was introduced by Mr. Archer, who substituted for the iodized paper a transparent film of iodized collodion spread upon glass, as the recipient of the negative picture. The process is thus rendered more certain and very much more rapid, at the same time the manipulation is simplified, whilst the positive pictures obtained by transference of the negative impression are much sharper in their outline. The operation requires to be conducted in a manner different from that which is practised when paper is employed. The following is the method to be pursued:— 1. To prepare the bath of nitrate of silver, take of nitrate of silver 300 grains, dissolve the salt in 2 ounces of distilled water, and add grain of iodide of potassium dissolved in half a drachm of water; then add drop by drop a solution of carbonate of potassium till a slight permanent turbidity is produced; afterwards add distilled water until the mixture measures 10 ounces; filter, and add 24 minims of glacial acetic acid.† 2. Preparation of Solution of Collodion.-A solution of iodized collodion, which is suitable for the formation of negative pictures, may be prepared as follows (Hardwich):-Take of rectified ether (sp. gr. 0·725), and of alcohol (sp. gr. from 0·805 to 0·815) each *For further details upon the subject of photographic printing, &c., the reader is referred to Hardwich's Manual of Photographic Chemistry. Ordinary nitrate of silver is apt to contain a trace of nitric acid, which it is desirable to neutralize, because an acid solution is much less sensitive to the action of light than a neutral one. It is still more important, however, not to have any alkaline reaction, and as carbonate of silver is slightly soluble in the nitrate, the addition of acetic acid is subsequently made to guard against this: the iodide of potassium is added in order to saturate the bath with iodide of silver; if this precaution were not taken, the film of iodized collodion would be liable to lose a portion of iodide of silver, since this salt is also somewhat soluble in a solution of nitrate of silver. A suitable pyroxylin for this purpose may be obtained in the following manner: Take of oil of vitriol, sp. gr. 1843, six fluid ounces; pure nitrate of potassium, finely powdered and dried, 31⁄2 ounces avoirdupois; water, I fluid ounce; dried cotton wool, 60 grains. Mix the acid and water, and add the nitre gradually, stirring between each addition, until the whole of the salt is dissolved. Suffer the mixture to cool to 150° or 145° F., then add the cotton wool in small tufts at a time, taking care to plunge the cotton completely beneath the surface; cover it, and allow it to stand for ten minutes. Then press out the acid with a glass rod as completely as possible, and throw the pyroxylin into a large volume of cold water, and wash for half an hour; afterwards soak it well in water for 24 hours; lastly, wring it out in a cloth, and dry at a heat not exceeding 100° F. The substance thus obtained is completely soluble in a mixture of ether and alcohol. It is essential to attend to the strength of the acid and to the temperature employed. DETAILS OF THE COLLODION PROCESS. 889 4 fluid drachms; soluble pyroxylin, from 4 to 6 grains; iodide of potassium or ammonium, 2 grains; iodide of cadmium, 2 grains. First dissolve the iodides in the alcohol, then add the pyroxylin, and lastly the ether. Agitate the materials well, set them aside for twenty-four hours, and then decant the clear liquid, which will retain sufficient sensitiveness to admit of being used even at the end of a month after its preparation. 3. Preparation of the Collodion Film.-In order to make use of this solution, a plate of glass cut to the size required for the camera (after being washed with a solution of potash to free it from grease, rinsed in water, dried, and wiped with a clean silk handkerchief), is to be held horizontally in the left hand, and a portion of the collodion is to be poured steadily on the middle of the glass, and by slightly inclining the plate in different directions, made to flow completely over the upper surface; the excess of the solution is immediately to be poured back into the bottle. 4. Exciting the Plate for the Camera.-The nitrate bath having been introduced into a trough of glass or of gutta percha sufficiently wide to allow the introduction of the glass plate upon which the collodion is spread-the prepared plate, within half a minute after the film has been poured off its surface, is introduced into the solution of nitrate of silver; in from two to three minutes' time it is thoroughly impregnated with iodide of silver, and when withdrawn from the bath it will exhibit a cream-coloured opalescence. These operations must be effected in a room illuminated by light admitted through a yellow blind, or by the light of a candle screened by yellow glass (1028). 5. Exposure in the Camera.-The prepared plate is to be immediately introduced into the slide of the camera, in which it is to be exposed to the object for a few seconds (from 3 or 4 to 30 or 40) according to the nature of the object and the intensity of the light. The slide is then withdrawn from the camera, and the plate, when examined in the darkened chamber, will not be found to exhibit any image. 6. Developing the Image.-A latent image, however, exists, and it may be developed by the use of a liquid prepared by dissolving 1 grain of pyrogallic acid, 10 minims of alcohol, and from 10 to 20 minims of glacial acetic acid, in an ounce of distilled water. Half an ounce or more of this liquid is to be poured over the plate immediately after its removal from the camera. The nega tive image which is thus gradually developed, will be more intense if immediately before using the pyrogallic solution an addition be made to it of the same solution of nitrate of silver as is employed 890 DEVELOPING THE IMAGE. in the bath, in the proportion of 2 drops to each drachm of the developing liquid. The exact reaction which occurs in this remarkable process is not known. The pyrogallic acid, however, is a substance which has a strong tendency to combine with oxygen; and under the conjoined action of iodide of silver and nitrate of silver (the presence of the latter salt in excess being necessary to the reaction) a portion of silver is reduced and is deposited upon those parts of the film which have been exposed to the action of light. Other solutions may be employed for developing the latent image. One which answers very well for this purpose consists of-crystallized sulphate of iron from 12 to 20 grains, glacial acetic acid 20 minims, alcohol 10 minims, and water 1 ounce. It is not, however, so well adapted for the production of intense negatives as the pyrogallic acid. When the picture is sufficiently distinct it must be washed with clean water, and fixed by immersing it in a solution of hyposulphite of sodium (1 part of the salt to 2 of water) till the cream-coloured iodide of silver is entirely removed. A solution of cyanide of potassium, containing from 2 to 12 grains of the salt in an ounce of water, may be substituted for the hyposulphite of sodium for the purpose of fixing the image. The picture is again to be thoroughly washed in clean water; it is allowed to dry, then heated before a fire until it feels slightly warm, and the film is protected from mechanical injury by covering it with a coat of transparent spirit-varnish by a manipulation similar to that employed in coating the plate with collodion. This varnished photograph may then be employed for procuring positive pictures by means of the sensitive paper prepared with chloride of silver upon Fox Talbot's plan (1016). By employing a neutral nitrate bath free from all organic matter, and a collodion which when iodized with iodide of potassium remains very nearly colourless, the sensitiveness of the film to the action of light may be so highly exalted, that moving objects, such as the waves of the sea, or a crowd of people, may be successfully depicted by the instantaneous action of light upon the plate. (1019) Albuminized Plates.-Niepce de St. Victor introduced the employment of glasses coated with albumin, prepared by beating up whites of eggs with 1 per cent. of iodide of potassium: the liquid is to be placed for 12 or 24 hours in deep vessels, to become clear, after which the supernatant liquid is to be poured upon glass so as to produce a uniform layer; it is then allowed to dry for 12 hours, and is fit for the bath of nitrate of silver. Albuminized glasses may be preserved for some weeks without PHOTOGRAPHIC ENGRAVING. 891 injury; they may be excited by means of Talbot's mixture of acetonitrate of silver with gallic acid (1017). The image is developed by means of a solution of gallic acid, after the plate has been exposed in the camera. (1020) Photographic Engraving and Lithography.—In the year 1827, Niepce published a process for obtaining pictures by the aid of light, the basis of which was the fact that the bitumen of Judæa, when exposed to the sun's rays, becomes insoluble in oil of lavender, whilst those parts which have remained in shadow preserve their solubility. This process has, with some modification, been applied by Niepce de St. Victor, the nephew of the inventor, to the production of engravings upon steel. Powdered asphalt and a proportion of pure bees-wax are dissolved in oil of lavender, and then mixed with an equal volume of benzol. The surface of the steel plate which is to be engraved is first carefully cleaned with whiting and water, after which a solution of hydrochloric acid in 20 parts of water is poured over it, and the plate is immediately washed and dried. The solution of bitumen is then poured upon the plate in a darkened chamber, and dried by the application of a gentle heat. A good positive photographic proof is now applied to the surface, covered with glass, and exposed for a short time to the action of diffused light. The exposed plate is next subjected to the action of a mixture of 3 parts of rectified naphtha and I of benzol; the parts which have not been exposed to light are gradually acted upon by this mixture. When the process of solution has proceeded far enough, the solvent is washed off with water, and the exposed parts of the plate are 'bitten in' with a mixture of 1 measure of nitric acid, sp. gr. 133, 2 measures of alcohol, sp. gr. 0844, and 8 measures of water. The plate is then submitted to the ordinary processes employed in aqua-tint engraving. An important modification of a process proposed by Poitevin for producing lithographs by the aid of photography, has been introduced by Mr. Osborne of Melbourne. The basis of this operation is the observation that a mixture of the anhydro-chromate of potassium and gelatin, when exposed to light, becomes insoluble in water. In order to apply this to practice, 800 grs. of gelatin and 440 of acid-chromate of potassium are dissolved in 8 ounces of warm water; when cooled to about 110°, 2 ounces of albumen from perfectly fresh eggs are added, and the whole is well mixed; the sheets of paper are then coated with this mixture on one side, hung up to dry in the dark, and are glazed by pressure. These operations are conducted in a room illuminated by yellow |