THEORY OF PHOTOGRAPHY The Sensitive Material.—If a photographic negative be ex amined under a microscope, it will be seen to consist not of a continuous homogeneous surface but of discrete, small silver grains occurring chiefly in clumps. (See Plate V., fig. 3.) These silver grains are derived from the structure of the original sensi tive emulsion. This is shown in Plate V., fig. 4, where we see that the developed grains correspond in general form and position to the undeveloped silver bromide grains from which they were produced.
If the emulsion is removed from a sensitive film and spread out under a microscope in a thin layer, it will be seen that the silver bromide is in the form of crystals, these being usually flat crystals, generally hexagonal or triangular in shape. (See Plate V., fig. 5). These crystals are very small„*,,, in. or less across ; there are great numbers of them in a film, several thousand million in a square inch, and their size depends upon the way in which the emulsion was made.
If an emulsion is made by adding a solution of silver nitrate to one of bromide in the absence of gelatine, the silver bromide formed will settle out in coarse flocks, but if a small quantity of gelatine be added to the solution, the silver bromide will be precipitated in a much finer form and will remain in suspension. In making an emulsion, therefore, gelatine is added to the bromide solution, and the mixture is brought to the proper temperature, then the silver nitrate solution is added, and the whole is heated.
When first mixed, the emulsion by transmitted light appears of a deep ruby colour. As heating is continued, the colour changes, becoming finally a bluish gray. Examination of the precipitate by means of X-rays has shown that the silver halide is crystalline from the beginning, the changes in colour and absorption being due merely to an increase in the size of the crystals. As this increase in size proceeds, the emulsion gains in sensitivity, but the chief increase in sensitivity occurs after the bulk of the gelatine has been added. After the emulsion is set it is cut up
by squeezing it through a sieve and then washed to remove the soluble salts. It is then further ripened to obtain the sensitiveness required.
The sensitiveness obtained depends to a very large extent upon the particular kind of gelatine used, some gelatine giving high sensitiveness, while other samples, apparently of the same chem ical and physical properties, will not give good sensitiveness how ever long the heating is continued. As will be shown later, this has been found to be due to a small impurity carried in the gelatine.
The crystals in an emulsion vary very greatly in size, their size and distribution being controlled by the conditions under which the emulsion was precipitated and digested. The sizes of grains which occur in an emulsion are directly connected with photographic properties of that emulsion.
When the grains cover a wide range of sizes, the emulsion is suitable for making negatives; it usually has a high degree of sensitiveness and a low degree of contrast. When, on the other hand, the grains are small and uniform, the emulsion has a high degree of contrast and a lower sensitiveness, such emulsions being used for positive film or lantern slide making.
If we suppose that the same amount of photo-chemical product (latent image) makes either a large or a small grain developable, then, other things being equal, the larger grain contributes a larger amount of silver to the image and thus gives more density for the same amount of light action than a smaller grain. For this reason alone, emulsions containing large grains would be more sensitive than those containing small grains, but experiment has shown that this is not the only factor and that, on the aver age, larger grains require less light action to become develop able than small grains. Further, it has been shown that the seat of this sensitiveness is concentrated in specks in the grains, and the sensitiveness of the grain depends upon the presence and the number of these sensitive specks.