However, the analytical methods based on a study of the structure diagrams given by X-rays indicate the presence of very small amounts of metal silver, of which the concentration increases gradually as the excitation becomes greater.' From our experimental knowledge of the latent image a certain number of facts are known of which the principal ones may be mentioned here. The latent image formed in a gelatine silver bromide emulsion still exists after the silver bromide has been changed into silver or after the silver bromide has been removed from the emulsion in the form of soluble salts (development after fixation, § 396). If increasing quantities of light act on different regions of a single sensitive film, there are obtained, after development under the same conditions, images which at first get more and more opaque up to a certain maximum and then begin to decrease in opacity (reversal). There after the latent image is no longer developable by ordinary means even for a considerable quantity of light incident on the emulsion 2 (solarization). A latent image having characteristics appar ently identical with those in the case of light can be obtained, at least with ripened emulsions, by the action of different reducing agents which have scarcely any action on an emulsion from which the nuclei have previously been removed in an oxidizing bath. If a sensitive emulsion is treated, after having been exposed to the light, with a solution of chromic acid, the latent image is more or less destroyed. This destruction, apparently complete with fine-grain emulsions, is incomplete in the case of those with large grains, the chromic acid thus being able to act only on the surface nuclei of the latent image, and not on those inside the grain. The latter become susceptible to development in cases where this is carried out after The latent image may be weakened or des troyed by exposure to red, and especially infra-red, light (Herschel effect, 1839), if the emulsion contains a soluble bromide. 4 This phenomenon is often better observed by long exposure and weak illumination than by short exposure to a light which is very intense. Also, it is facilitated by the presence in the emulsion of organic desensitizers (safranine), or some mineral salts (traces of copper salts). By such means a positive may be obtained by exposing to red light, through another positive, a plate which has been uniformly fogged (Liippo-Cramer, 1927.) 198. The change which takes place in silver bromide during a short exposure to light is so minute, that it was formerly thought to be a modification of the physical state or an allotropic transformation. These hypotheses, as well as those which assume an oxidation of the silver bromide, are ruled out by the fact that the same phenomena can be produced by means of very weak solutions of feebly-reducing substances, e.g. sodium arsenite.
For a long time it was believed that the violet coloured or reddish-coloured products obtained by light-action on silver bromide and silver chloride were definite substances in which the silver was combined with half the quantity of chlorine or bromine with which it is combined in the normal silver chloride and silver bromide. It has been shown beyond dispute that these silver sub-halides have no separate existence, and are only " solid solutions " of colloidal silver in the normal silver chloride and silver bromide. Thus the hypothesis which considers the latent image as formed of very small particles of silver sub-bromide can no longer hold.
The latent image is probably silver adsorbed to the silver bromide, as Carey-Lea suggested in 1887 and as supported by experiments of Luppo-Cramer since 19°6. This adsorbed silver possesses, however, certain properties which are different from those of completely free silver. 2 199. Different Actions on the Photographic Emulsion. The photographic emulsion is sensi tive not only to ordinary visible light but to all radiations of shorter wavelengths, especially the ultra-violet 1, X-rays and similar radiations emitted by various radio-active bodies, and even in certain conditions by supersonic waves.
Images can also be obtained by static electric discharge in contact with the film.
Different mechanical actions, such as sliding (friction of a blunt point), rubbing, etc., all cause the emulsion to become developable (abrasion of the film), and at the same time make the emulsion insensitive to the subsequent action of light (0. Bloch, 1915). Just like light, these different actions, if they are extremely intense, give rise to phenomena similar to reversal and solarization (§ 197). These reversals can generally be obtained much more easily if two different actions, or two different degrees of the same action, are made to act succes sively on the emulsion. Thus it is that an abrasion can give rise to black marks in the whites of an image and white marks in the black.
Contact with the emulsion of a great num ber of reducing solutions (sodium arsenite, stannous chloride, etc.), or of gaseous reducers (sulphuretted hydrogen, hydrogen phosphide) causes an intense fog.
Special mention must be made of the action of hydrogen peroxide, 3 first noticed by W. _T Russell in 1899, and more recently studied by Liippo-Cramer, W. Clark, and others.
The contact of a sensitive film, in which reduction nuclei have been formed by sufficient ripening, with neutral or acid solutions of hydrogen or with the vapours evolved by these solutions, gives rise to a series normally into the construction of cameras and plate holders. This explains the fog which is often found on a plate which has been left a long time in its holder, especially in one of recent construction. These phenomena cease as soon as the surface oxidation is completed. I A specially interesting case is that of the of phenomena which are all analogous to those arising from the action of light (such as forma tion of a developable image, reversal, and solarization). Ozonized air has almost the same characteristics.
A very large number of organic substances, and some metals, oxidize slowly in moist air and give rise to traces of hydrogen peroxide (or ozone), which can act from a distance on sensi tive emulsions and cause the formation of fog. Such is the case, for example, with different woods (especially resinous woods), turpentine, and other vegetable essences, numerous resins, base resin varnishes, lignites, and, amongst metals, zinc, and, to a less degree, aluminium and magnesium.
The reader will notice that several of the substances that have just been mentioned enter action of paper on the emulsion. Niepce de St. Victor (1857) attributed the action on a photo graphic plate of a piece of paper which had previously been exposed to the sun to an invisible ray, comparable with a phosphores cence. It is now known that this action is due to the formation of hydrogen peroxide by the oxidation of the size in moist air, and that it can be avoided by all bodies which destroy the hydrogen peroxide. Similar complex pheno mena may be noticed when a paper, having printed or written letters on it, is placed in con tact with the film ; according to the nature of the ink, the latter may exert a more intense action than the paper, or, on the contrary, the metallic salts in the ink may desensitize the emulsion, so that, after a sufficiently long time in contact, the image of the text may appear, on development, deep grey on a clearer ground, or light grey on a dark ground.' 200. Various Actions on the Latent Image. If in identical conditions various parts of one and the same plate or film are exposed at known intervals of time running into hours, the nega tive developed immediately after the last expo sure will show different densities in the different parts. As a rule, the oldest latent image pro duces a somewhat denser image, this spon taneous evolution of the latent image being at first rapid and then slower and More over, it is not the same for latent images pro duced by different radiations.