in 1850, on the introduction of collodion (q.v.), a very convenient vehicle on account of the facility with which the plates are prepared, and also because it is a substance as a rule totally unaffected by silver nitrate, which is not the case with other organic substances. Thus albumen forms a definite silver compound, as do gelatin, starch and gum. The employment of collodion was first suggested by Le Gray, but it remained for Frederick Scott Archer of London, closely followed by P. W. Fry, to make a really practical use of the discovery. When collodion is poured on a glass plate it leaves on drying a hard transparent film which under the microscope is slightly reticulated. Before drying, the film is gelatinous and perfectly adapted for holding in situ salts soluble in ether and alcohol. Where such salts are present they crystallize out when the film is dried, hence such a film is only suitable where the plates are ready to be immersed in the silver bath. As a rule, about five grains of the soluble gun-cotton are dissolved in an ounce of a mixture of equal parts of ether and alcohol, both of which must be of low specific gravity, ·725 and ·805 respectively. If the alcohol or ether be much diluted with water the gun-cotton (pyroxylin) precipitates, but, even if less diluted, it forms a film which is “crapey” and uneven. Such was the material which Le Gray proposed and which Archer brought into practical use. The opaque silver plate with its one impression was abandoned; and the paper support of Talbot, with its inequalities of grain and thickness, followed suit, though not immediately. When once a negative had been obtained with collodion on a glass plate the image showing high lights by almost complete opacity and the shadows by transparency (as was the case, too, in the calotype process)-any number of impressions could be obtained by means of the silver-printing process introduced by Fox Talbot, and they were found to possess a delicacy and refinement of detail that certainly eclipsed the finest print obtained from a calotype negative. To any one who had practised the somewhat tedious calotype process, or the waxed-paper process of Le Gray with its still longer preparation and development, the advent of the collodion method must have been extremely welcome, since it effected a saving in time, money and uncertainty. The rapidity of photographic action was much increased, and the production of a different character of pictures thus became possible.
We give an outline of the procedure. A glass plate is carefully cleaned by a detergent such as a cream of trioli powder and spirits of wine (to which a little ammonia is often adci)ed), then wiped with a soft rag, and finally polished with a silk handkerchief or chamois leather. A collodion containing soluble iodides and bromides is made to flow over the plate, all excess being drained off when it is covered. A good standard formula for the collodion is—55 grains of pyroxylin, 5 oz of alcohol, 5 oz of ether; and in this liquid are dissolved 212 grains of ammonium iodide. 2 grains of cadmium iodide and 2 grains of cadmium bromide. When the collodion is set the plate is immersed in a bath of silver nitrate—a vertical form being that mostly used in England, whilst a horizontal dish is used on the continent of Europe—a good formula for which is 350 grains of silver nitrate with 10 oz. of water. The plate is steadily lowered into this solution, and moved in it until all the repellent action between the aqueous solution of the silver and the solvents of the collodion is removed, when it is allowed to rest for a couple of minutes, after which period it is taken out and placed in the dark slide ready for exposure in the camera. After undergoing proper exposure the plate is withdrawn, and in a room lighted with yellow light the developing solution is applied, which originally was a solution of pyrogallic acid in water restrained in its action by the addition of acetic acid. One of the old formulae employed by P. H. Delamotte was 9 grains of pyrogallic acid, 2 drachms of glacial acetic acid and 3 oz. of water. The image gradually ap ars after the application of this solution, building itself up from the silver nitrate clinging to the film, which is reduced to the metallic state by degrees. Should the density be insufficient a few drops of silver nitrate are added to the pryogallic acid solution and the developing action continued.
In 1844 Robert Hunt introduced another reducing agent, which is still the favourite, viz. ferrous sulphate. By its use the time of necessary exposure of the plate is reduced and the image develops with great rapidity. A sample of this developing solution is 20 grains of ferrous sulphate, 20 minims of acetic acid, with 1 oz. of water. This often leaves the image thinner than is requisite for the formation of a good print. and it is intensified with pyrogallic acid and silver. Other intensifiers are used to increase the deposit on a plate by means of mercury or uranium, followed by other solutions to still further darken the double salts formed on the film. Such intensifying agents have to be applied to the image after the plate is fixed, which is done by a concentrated solution of hyposulphite of soda or by potassium cyanide, the latter salt having been first introduced by Martin and Marc Antoine Augustin Gaudin in 1853 (La Lumiére, April 23, 1853). Twenty-five rains of potassium cyanide to one ounce of water IS the strength of time solution usually employed. The reaction of both these fixing agents is to form with the sensitive salts of silver double hyposulphites or cyanides, which are soluble in water and salt. The utility of bromides in the collodion process seems to have been recognized in its earliest days, Scott Archer (1852) and R. J. Bingham (1850) both mentioning it. We notice this since as late as 1866 a patent-right in its use was sought to be enforced in America, the patent being taken out by James Cutting in July 1854.
Positive Pictures by the Collodion Process.—In the infancy of the collodion process it was shown by Horne that a negative image could be made to assume the appearance of a positive by whitening the metallic silver deposit. This he effected by using with the pyrogallic acid developer a small quantity of nitric acid. A better result was obtained by P. W. Fry with ferrous sulphate and ferrous nitrate, whilst Hugh Diamond gave effect to the matter in a practical way. F. Scott Archer used mercuric chloride to whiten the image. To Robert Hunt, however, must be rewarded the credit of noticing the action of this salt on the image (Phil. Trans., 1843). The whitened picture may be made to stand out against black velvet, or black varnish may be poured over the film to give the necessary black background, or, more recently, the positive pictures may be produced on japanned iron plates (ferrotype plates) or on japanned leather. This process is still occasionally practised by itinerant photographers.
Moist Collodion Process.—It is seen that for the successful working of the collodion process it was necessary that the plate should be exposed very shortly after its preparation; this was a drawback, inasmuch as it necessitated taking a heavy equipment into the field. In 1856, Sir William Crookes and J. Spiller published in the Philosophical Magazine a process whereby they were enabled to keep a film moist (so as to prevent crystallization of the silver nitrate) several days, enabling plates to be prepared at home, exposed in the field, and then developed in the dark room. The plate was prepared in the usual way and a solution of zinc nitrate and silver nitrate in water was made to flow over it. The hygroscopic nature of the zinc salt kept sufficient moisture on the plate to attain the desired end. Various modifications in procedure have been made.
Dry Plates.—It would appear that the first experiments with collodion dry plates were due to Marc Antoine Augustin Gaudin. In La Lumiére of the 22nd of April and the 27th of May 1854 he describes his researches on the question; whilst in England G. R. Muirhead, on the 4th of August 1854, stated that light acts almost as energetically on a dry surface as on a. wet after all the silver has been washed away from the former previous to desiccation. J. M. Taupenot, however, seems to have been the first to use a dry-plate process that was really workable. His original plan was to coat a plate with collodion, sensitize it in the ordinary manner, wash it, cause a solution of albumen to flow over the surface, dry it, dip it in a bath of silver nitrate acidified with acetic acid, and wash and dry it again. The plate was then in a condition to be exposed, and was to be developed with pyrogallic acid and silver. In this method we have a double manipulation, which is long in execution, though perfectly effective.
A great advance was made in all dry-plate processes by the introduction of what is known as the “alkaline developer,” which is, however, inapplicable to all plates on which silver nitrate is present in the free state. The developers previously described, either for collodion or paper processes, were dependent on the reduction of metallic silver by some such agent as ferrous sulphate, the reduction taking place gradually and the reduced particles aggregating on those portions of the film which had been acted upon by light. The action of light being to reduce the silver iodide, bromide or chloride, these reduced particles really acted as nuclei for the crystallized metal. It will be evident that in such a method of development the molecular attraction
