One of the beet modca of procedure now adopted is the following:— Take a plate of silvered copper and polish it by moans of tripoli powder and oil of lavender or roaemary, applied by cotton velvet, finishing the polish by clew, cotton velvet alone. Then expose the silver to a mixture of iodine and pure sand in such a manner that the vapour of the iodine shall act equally upon the surface of the silver plate, to which it imparts a coating which is Seen to be coloured when examined by light reflected from any white surface, a piece of paper for example. As soon as the plate has assumed an orange-yellow colour it is removed, and then exposed to the vapour issuing from a peculiar red compound of bromine with lime, called " bromide of lime." Over this it absorbs bromine, and assumes a rose tint; and as soon as this shade of colour has been obtained, the plate must be removed and again exposed to the iodine vessel until the rose colour has deepened into a plum tint. The plate is then reedy for exposure in the camera obscure. No time can be etated for these vitriolic, exposures, a.9 temperature influences the results. A few eeeonds in each case suffice. plate must be prepared in a room which can be darkened, the light of a candle, or that obtained through yellow glass, being siono used at the last iodising, end in mime of the subsequent operations. After exposure in the camera, the plate is exposed to the vapour of mercury for a few minutes, the mercury being at a tempe rature of about 180' Fair. Here the picture is developed by the action of the mercury upon the bromo-iodised surface, the mercury being, it is believed by some, deposited upon the plate in proportion to the amount of light which fell upon its aurfece during its exposure in the camera. On its removal from the mercury box the plate is partially fixed by washing its surface with a strong solution of hypo sulphite of soda. The final fixation is effeotod by boiling upon the plate a solution of a double salt, called hyposulphite of soda and gold. The image is now fixed upon the plate, sad may be coloered by brushing over it colours in very fine powder. The image sheet(' be kept so as to exclude the vapours of an impure atmosphere, such as is usually found in large towns. Sulphuretted vapours will at once darken the light part of the image. The film of stain may, however, generally be removed by a solution of cyanide of potassium. The Daguerreotype is still unsurpassed in delicacy of detail.
Having given an account of the Daguerreotype, we might proceed to relate the history of Mr. Fox Talbot's researches, which led to the invention of the first successful process in photography on paper ; but as these will be found detailed in Mr. Talbot's work • The Pencil of Nature,' and in the specifications of his patents, we prefer to pass at once to consider a process which has now almost superseded all others, and which certainly sprang out of Mr. Fox Talbot's discoveries. Mr. Talbot used iodide of silver with nitrate of silver in excess, upon paper, for the purpose of procuring an image which remained latent until developed by a solution of gallic acid. The process now used, and called the Collodion process, of Mr. Scott Archer, consists in the use of a film, on glass, of collodion, containing also iodide of silver with an excess of nitrate, the development being accomplished by pyro-gallic acid, or by sulphate of iron, the use of which we owe to Mr. R. Hunt, in the place of gallic acid. The analogy is complete, but the latter materials give a greater delicacy of result.
Collodion is made by dissolving in ether and alcohol cotton-wool which has been altered in its properties by treatment with strong acids. The following is a good mode of proceeding, and is due mainly to the researches of Mr. Hatless.* Take of pure nitrate of potash in fine powder 510 grains, of oil of vitriol (specific gravity about) 151 drachms, of water 14 drachm ; stir together, and when at a tempe rature of from 150* to 155° Fahr. add, bit by bit, 15 grains of cotton wool to each ounce of the acid mixture. Allow the cotton to soak for four or five minutes, and then wash it many times in water until it is quite free from acid. Then, to make the collodion, take 9 grains of the dry cotton, and add 6 drachms of pure ether (sp. gr. .725 to .730), and 2 drachms of pure alcohol (sp. gr. .818 to '820). The cotton should at once dissolve. In another bottle prepare what is called the " iodising solution," by taking alcohol (sp. gr. .818 to •S20) one ounce, iodide of potassium 12 grains, iodide of cadmium 4 grains; dissolve the salts in the alcohol, and keep the solution for use. To make "iodised collodion,"
mix 6 drachms of collodion with 2 drachms of the iodising solution ; this mixture changes by keeping, and should therefore be made only in moderate quantities. Having prepared the iodised collodion, a plate of glass is covered with it by pouring a quantity on the centre of the plate, and then allowing the liquid to flaw to the corners in such a way that the glass shall be uniformly covered; the excess is then run off at one corner into a bottle set apart for the purpose. After a few seconds the film of iodised collodion is sufficiently firm to be fit for immersion (in the dark) in what is called the " uitrato bath." This bath is made by dissolving 30 grains of nitrate of silver in one ounce of distilled water. The nitrate of silver should be pure, and free from excess of nitric acid, and it should be saturated, when in solution, with iodide of silver : a little acetic acid, too, may be added. The plate is immersed in this bath for a few minutes, drained,and then exposed in the camera obscura. To develops the image, a solution of pyrogallic acid con taining one grain of the salt to one ounce of water acidulated by 20 drops, or minims, of glacial acetic acid is poured upon the plate. If the imago it not intense enough, a little weak solution of nitrate of silver must be added. The image obtained Is fixed by a strong solution of hyposulphite of soda, in which it is immersed; or, instead, a weak solution of cyanide of potassium may be poured upon the plate, and left there until the yellow Olin of iodide of silver disappears. The plate is then washed and dried, and protected by a film of varnish, a spirit varnish being usually preferred for this purpose.
The picture thus obtained is, as in the calotype or Talbotype pro CCU of Mr. Fox Talbot, a negative one—that is to say, a picture having its light and shade reversed ; though by modifying the collodion process direct positives may be at once obtained : a good uegative, however, is a more valuable acquisition. In order to obtain copies correct in light and shade and position, a positive has to be made. There are many processes by which this can be done ; but we will here give only one, which answers perfectly. Take the white of a fresh egg and beat up with every fluid ounce of it 12 grains of common salt : remove the froth thus obtained, and continue boating until all has become froth. heave this froth to itself, and the greater part of the white of egg will become again liquid. Pour this liquid into a flat shallow dish, and upon it place carefully, so as to exclude bubbles of air, a sheet of thin paper (French paper is usually chosen) ; leave the paper for two or three minutes floating, so as to coat only one side with the "salted albumen," as the white of egg mixture is now called. Then carefully remtve the sheet, and pin it up by a corner to dry. This operation can be carried on hi daylight, To make this paper sensitive, it is floatad upon a solution of nitrate of silver containing 100 grains of nitrate to one ounce of water. Hero it is left for a minute, and then la moved and suspended to dry. This last operation must be performed in a room lighted by yellow light, as in the case of the daguerreotype and collodion final preparation. Upon the•dry paper the negative picture is placed face to face, and the whole exposed in a proper pressure-frame' to the sun or to daylight, or to the electric light for ten minutes or a quarter of an hour by night. After a few minutes the picture is found printed, and must next be fixed by immersion in a solution of hyposulphite of soda, one part of the salt in four parts of water. Ten or fifteen minutes' immersion would suffice to fix the picture ; but in order to produce an agreeable tint of colour, a longer immersion is had recourse to, with the addition to the '• fixing bath " of a few grains of a neutral solution of chloride of gold. After several hours' immersion in this batb, the picture is removed, and washed repeatedly with plenty of water; hot distilled water being used at last. This fixing bath is made fresh for each day's work. The pictures, when washed and dried, may be mounted on cardboard by means of starch, gum, or gelatine; • paste is supposed to injure the picture under some circumstances. Exposure of the finished picture to impure atmospheric vapours, and to damp, is to be avoided. Mr. Malone has advised that the picture should be heated in a solution of caustic potash in order to secure its greater permanency. He also insists on the injurious action of sulphur in certain forms upon the print.