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image, silver, light, action, formed, bromide, iodide and normal

LATENT IMAGE.—The name given to the "latent " or invisible image, resulting from the impact of light upon the sensitive photographic plate. This image, although invisible to the eye, quickly becomes perceptible when treated with a reducing agent termed a developer.

The theory of the latent image is surrounded in mystery. Many attempts to solve it have been made by a number of eminent chemists and scientists. The results of numerous experiments regarding the acton of the light upon silver compounds show that when in a vacuum pure silver haloids do not become decomposed by light at all, but if moisture be pres ent a change takes place with the silver bromide or chloride, and a dark-colored compound containing a lower percentage of the halogen than contained in the original salt, a substance also being formed having the power of liberating iodine from potassium. The dark solid product has never been clearly explained. It is generally considered to be a sub-bromide or sub-chloride, but many scientists dispute this.

One of the first theories of the latent image was that the action of the light upon the sensitive salts was to set up a vibratory motion. Although this theory is still considered the correct one by a few, the experiments of Abney, Bothamley, Meldola, Carey Lea, and others, clearly prove it to be erroneous.

Chemically the action of the light is expressed thus— Silver Bromide. Silver sub-bromide. Bromine.

2AgBr + light = Ag2Br + Br The following remarks are mostly taken from an excellent summary (by Mr. C. P. Both amley, F. I. C.)* of what is already known about the matter: The physical theory of the latent image assumes that the energy of the light rays is trans ferred to the molecules of the silver haloid, which are thereby thrown into a state of unstable equilibrium such that the compound is reduced to the metallic state by reagents which, under normal conditions would have no action on it. The chief evidence in support of this view was the observation that the latent image spontaneously disappeared. There is, however, a consider able amount of evidence to show that the latent image does not fade spontaneously, but, in all cases where its disappearance is observed, it is destroyed by the action of atmospheric impurities, or by secondary reactions with substances retained in the film. Carey Lea showed that silver iodide will absorb free iodine, and hence the gradual disappearance of the latent image on Da guerreotype plates, or any of the films obtained by treating silver with an iodizing agent, may be traced to the fact that the silver iodide has absorbed an excess of iodine during its prepara tion, and this iodine gradually acts upon the product of the action of light and re-converts it into normal silver iodide.

Analogy would lead us to expect that if the silver haloid were thrown into an unstable condition by the action of waves of light, it would gradually and somewhat rapidly return to its normal condition when the disturbing cause ceased to act. Gelatino-bromide plates have been kept two or three years after exposure, without any reduction in the character of the image on development. It is in the highest degree improbable that that a mere condition of unstable equi librium would persist for so long a time, and these facts, combined with the fact that the latent image is destroyed by reagents of a particular kind, but not by others, seems to be conclusive against the physical theory.

All evidence indicates that the formation of the invisible image is a photo-chemical opera tion, the composition of the material forming the image being different from that of silver bro mide. The difference between the formation of a visible image and a latent image is a difference in degree, and not in kind.

Bothamley further remarks that it seems that it has not been clearly recognized that the formation of a developed photographic image takes place in three distinct stages, which may, and probably do, differ considerably in the nature of the changes which occur. First, we have the latent photo image formed solely by the action of light, and therefore of photo-chemical origin; secondly, we have what he calls the primary or fundamental image, formed by the latent image by the action of the developer, and, therefore; partly of photo-chemical and partly of chemical origin; and, thirdly, the developed image, formed by the action of the developer from the primary image and the unaltered silver bromide in the film. This last process is probably mainly electro chemical. It has not yet been definitely proved that the material composing the latent image is identical in composition with the visible products formed by the more prolonged action of light, although the various known facts furnish very strong proof that this is the case.

The results of the experiment seem to be in a point being reached that the latent image is a photo-mechanical reduction product containing a lower proportion of halogen than normal silver chloride, or bromide, or iodide, and much more easily reduced.to the metallic state. The problem of its composition and constitution still remains to be solved.