Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/863

 PHOTOGRAPHY 827 example of which we have in the preparation of a gelatin plate. In this case the gelatin wraps up the particles of the silver compound in a colloidal sheath, as it were, and the developing solution only gets at them in a very gradual manner, for the natural tendency of all such reducing agents is to attack the particles on which least work has to be expended. In the case of bromide of silver the developer has only to remove one atom of bromine, whereas it has to remove two in the case of sub-bromide of silver. The sub-bromide formed by light and that sub sequently produced in the act of development are therefore reduced. A large proportion of gelatin compared with the silver salt in a film enables an alkaline developer to be used without any chemical restrainer ; but when the gelatin bears a small proportion to the silver such a restrainer has to be used. With collodion films the particles of bromide are more or less unenveloped, and hence in this case some kind of chemical restrainer is absolutely necessary. We may say that the organic iron developers require less restraining in their action than do the alkaline developers. Alkaline development was first used by Major Russell in a dry-plate process in which the collodion was merely bromized by means of bromides soluble in alcohol. The plate was prepared by immersion in a strong solution of silver nitrate and then washed and a preservative applied. The last-named agent executes two functions, one being to absorb the halogen liberated by the action of light and the other to preserve the film from atmospheric action. Tannin, which Major Russell employed, if we mistake not, is a good absorbent of the halogens, and acts as a varnish to the film. Other collodion dry-plate processes carried out by means of the silver-nitrate bath were very numerous at one time, many different organic bodies being also employed. In most cases ordinary iodized collodion was made use of, a small percentage of soluble bromide being as a rule added to it. When plates were developed by the alkaline method this extra bromide induced density, since it was the silver bromide alone which was amen able to it, the iodide being almost entirely unaffected by the weak developer which was at that time in general use. One of the most successful bath dry-plate processes was introduced by Mr. R, Manners Gordon and was a really beautiful process. The plate was given an edging of albumen and then coated with ordinary iodized collodion to which one grain per ounce of cadmium bromide had been added. It was kept in the silver-nitrate bath for ten minutes, after which it was washed thoroughly. The following preservative was then applied : ( Gum arable 20 grs. 1. -j Sugar candy 5 ,, (Water 6 dr. 2 f Gallic acid 3 grs. These ingredients were mixed just before use and, after filtering, applied for one minute to the plate, which was allowed to drain and set up to dry naturally. Great latitude is admissible in the exposure ; it should rarely be less than four times or more than twenty times that which would be required for a wet plate under ordinary circum stances. The image may be developed with ferrous sulphate restrained by a solution of gelatin and glacial acetic acid, to which a solution of silver nitrate is added just before application, or by the following alkaline developer : -. ( Fyrogallic acid 96 grs. Alcohol 1 oz. c) ( Potassium bromide 12 grs. 1 Water 1 oz. o ( Ammonium carbonate 80 grs. Water 1 oz. The development of the image requires 6 minims of No. 1, | drachm of No. 2, with 3 drachms of No. 3. If properly exposed the image appears rapidly and gradually gains in intensity, and when all action from the developer ceases the plate is washed and further intensified with pyrogallic acid and silver as is a wet plate. The image is finally fixed in sodium hyposulphite. In photographic processes not only has the chemical condition of the film to be taken into account but also the optical. When light falls on a semi-opaque or translucent film it is scattered by the particles in it and passes through the glass plate to the back. Here the rays are partly transmitted and partly reflected, a very small quantity of them being absorbed by the material of the glass. Theory points out that the strongest reflexion from the back of the glass should take place at the vertical angle. In 1875 Abney investigated the subject and proved that practice agreed with theory in every respect, and that the image of a point of light in development on a plate was surrounded by a ring of reduced silver caused by the reflexion of the scattered light from the back surface of the glass, and that this ring was shaded inwards and outwards in such a manner that the shading varied with the intensity of the light reflected at different angles. To avoid &quot; halation,&quot; as this phenomenon is called, it was usual for photographers to cover the back of their dry plates with some material which should be in optical contact with it, and which at the same time should absorb all the photographically active rays, and only replace those which were incapable of reducing the silver salt. This was called &quot; backing a plate.&quot; Collodion Emulsion Processes. In 1861 Bolton and Collo- Sayce published the germ of a process which revolutionized dion photographic manipulations, and by a subsequent substi- e ul! tution of gelatin for collodion gave an impetus to photo- graphy which has carried it to that state of perfection at which it has arrived at the present time (1884). In the ordinary collodion process it will be recollected that a sen sitive film is procured by coating a glass plate with collodion containing the iodide and bromide of some soluble salt, and then, when set, immersing it in a solution of silver nitrate in order to form iodide and bromide of silver in the film. The question that presented itself to Bolton and Sayce was whether it might not be possible to get the sensitive salts of silver formed in the collodion whilst liquid, and a sensitive film given to a plate by merely let ting this collodion, containing the salts in suspension, flow over the glass plate. Gaudin had attempted to do this with chloride of silver, and later G. W. Simpson had suc ceeded in perfecting a printing process with collodion con taining chloride of silver, citric acid, and nitrate of silver ; but the chloride until recently has been considered a slow working salt, and nearly incapable of development. Up to the time of Bolton and Sayce s experiments iodide of silver had been considered the staple of a sensitive film ; and, though bromide had been used by Major Russell and others, it had not met with so much favour as to lead to the omission of the iodide. At the date mentioned the suspension of iodide of silver in collodion was not thought practicable, and the inventors of the process turned their attention to bromide of silver, which they found could be secured in such a fine state of division that it remained suspended for a considerable time in collodion, and even when precipitated could be resuspended by simple agita tion. The outline of the method was to dissolve a soluble bromide in plain collodion, and add to it drop by drop an alcoholic solution of silver nitrate, the latter being in excess or defect according to the will of the operator. To prepare a sensitive surface the collodion containing the emulsified sensitive salt was poured over a glass plate, allowed to set, and washed till all the soluble salts result ing from the double decomposition of the soluble bromide and the silver nitrate, together with the unaltered soluble