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Effect of Mercury Vapor Upon the Latent Image
So far only the effect of mercury vapor upon unexposed photographic emulsions has been considered. Theoretical considerations led to the discovery that the effect of mercury vapor upon the latent image is even greater than it is upon the unexposed emulsion. This action may probably be better described by the expression intensification of the latent image, as it has been applied to similar processes utilizing hydrogen peroxide.
In Fig. 3, A is the characteristic curve of an untreated emulsion; B is for the same emulsion dry-hypersensitized by mercury before exposure; and C is for the same emulsion, but in this case the mercury vapor treatment took place after exposure — in other words, the latent image has been intensified after exposure but before development. From the curves it can be seen that the
effect of the mercury upon the latent image is distinctly greater than it is upon
the unexposed emulsion. However, except for the difference in intensity of the effect, the characteristic behavior is in both cases the same. The characteristic curve of the intensified latent image, as can be seen in Fig. 3, also runs almost parallel to the original characteristic curve. The stability of intensification of the latent image is limited as to the length of time between treatment and development, as is the hypersensitization of the unexposed emulsion as to time between treatment and use.
The treatment of the exposed film with mercury vapor to intensify the
latent image can be done exactly in the same manner as has been described
for dry hpyersensitization. It is, therefore, possible to correct an underexposed picture by treating the undeveloped film with mercury vapor for a certain length of time, provided, of course, underexposure is known or suspected. After the treatment, the film is developed as usual, and will produce a negative similar to one exposed with 100 to 150 per cent more light. Tightly wound rolls in cans can be hypersensitized if sufficient time, generally six to eight days, is allowed. Due to the relatively slow action, good penetration to all the layers of emulsion is achieved, and the effect is more or less uniform throughout
Practical Applications
Within the scope of this paper it is possible to describe only very briefly how the material should be handled to obtain the best results. As a matter of fact, it would be very difficult to give exact formulas. Fortunately it is not necessary to do so; because of the slowness of the effect, the time of treatment, and the mercury vapor concentration do not have to be very exact. As a general rule, loose and unwrapped material should be treated from 30 to 40 hou rs at room temperature, while wrapped and spooled materials require treatment for seven to ten days, in a mercury vapor concentration created, for instance, by 0.5 gram of mercury in a 1000-ft. film container. In place of liquid mercury, of course, all compounds, amalgams, such as silver amalgams, and other preparations that emit mercury vapors can be used for dry hypersensitization or for intensifying the latent image.
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