EXPOSURE. The exposure of a sensitive surface to light, either in the camera or pressure frame, is one of the most important features of every photographic process ; and we shall endeavour to describe in this place the effects due both to over and under expo sure in the principal processes, and as far as possible to account for them satisfactorily.
The simplest case is that of sun-printing on chloride paper. Here a sensitive surface consisting of chloride of silver, nitrate of silver, and organic matter is exposed to direct light until a visible image of the required strength is obtained. The effect of light on such a surface is to decompose it and produce a dark material composed of subchloride of silver, together with a compound of suboxide of silver and organic matter. When the image thus obtained, and which is of a purple brown tint, is put into the bath of hypo sulphite of soda, in order to fix it by removing the unaltered chloride of silver, the purple subchloride in the image is also decomposed into chloride of silver and metallic silver, and the former dissolved out. The fixing bath therefore not only removes the superfluous chloride in the paper, but also enfeebles the dark tint of the shadows of the picture, by changing it from a deep purple to a red or brown. When sufficient allowance is not made for this change which occurs in fixing a print, it is of course "under-exposed" or "under printed." When, on the other hand, the exposure is carried too far, the reduced organic compound of silver presents a metallic lustre, or bronzed appearance, and the details in the shadows are obliterated, and buried beneath a crust which is not easily removed. in the fixing bath.
In printing by development on chloride paper, when the print is under-exposed, the details in the lights cannot be brought out until the development has been carried so far as either to stain the paper all over, or bury the details in the shadows. When the print is over-exposed, the details in the lights come out so rapidly in the gallic acid, and darken so quickly, that the development has to be stopped before the blacks have time to acquire sufficient depth and vigour. The picture is therefore red and feeble, and the lights and shades do not exhibit sufficient contrast.
It will be observed that in printing on chloride paper by clirect light, the longer the exposure the darker the picture becomes, up to a certain point, and then the reduced material afterwards assumes a metallic appearance. So also in development printing on chloride paper, the longer the exposure and the longer the development the darker the blacks become, up to a certain point, and then, as in the other case, they assume a metallic appearance. By carrying the
process too far, the same effect happens in both cases. And not only so, but this effect happens in every other photographic process with metallic salts. Excessive exposure produces an amount of reduction which defeats its object by exhibiting the metallic or solarized appearance in the case of a positive print, or, in the case of a negative, that metallic condition of the reduc,ed salt which interferes with its property of becoming a centre of attraction for the matter which should accumulate upon it from the developer.
But in these remarks we are anticipating the cases of the Collodion and Daguerreotype processes, in which iodide of silver takes the place of chloride. In these cases a curious effect occurs through over-exposure, which has been called, we think improperly, the " reverse action of light." In the Collodion process the sen sitiveness of the film is due to the presence of an excess of free nitrate of silver, which it is impossible to remove by any amount of washing with water, because it probably combines with the organic matter of the film. This free nitrate, existing in minute quantity in presence of iodide of silver, is decomposed by light with extra ordinary rapidity, and when not over-exposed and rendered metallic, it fiunishes the proper centres of attraction for the developing pyro gallo-nitrate. But if rendered metallic by over-exposure, it no longer acts in this way, and therefore no dark material is precipi tated upon it from the developer. In order, therefore, to gain the greatest intensity in the image, there is a certain amount of exposure which must not be exceeded.
In the daguerreotype process over-exposure produces in the iodized surface an effect which is shown by the solarized part look ing blue, or sometimes quite metallic. In an over-exposed collodion positive a very similar effect of blueness is produced, which shows the analogy existing between the prooesses.
It appears, therefore, that by over-exposure not only is more material reduced, but the reduction of the atoms which are first acted on by light is canied to such a stage, that they look metallic instead of black in positive prints, and in negatives cease to become fit centres of attraction for the production of a picture by a de veloping or intensifying process.