Another cutting reducer is permanganate. The permanganates are very strong oxidizing agents, and if a solution of perman ganate containing sulphuric acid is applied to a negative, it will oxidize the silver to silver sulphate, which is sufficiently soluble in water to be dissolved.
B. Proportional reducers are those which act on all parts of the negative in proportion to the amount of silver present ; hence they exactly undo the action of development, since during development the density of all parts of the negative increases proportionally. A correctly exposed but over-developed negative should be re duced with a proportional reducer. Unfortunately, there are no single substances which form exactly proportional reducers, but by mixing permanganate, which is a slightly cutting reducer, with persulphate, which is a flattening reducer, a proportional reducer may be obtained.
C. In order to have a flattening reducer, we require one which acts very much more on the heavy deposits than on the light de posits of the negative and which will consequently reduce the highlights without affecting the detail in the shadows. Only one such reducer is known, and this is ammonium persulphate. Am monium persulphate is a powerful oxidizing agent and attacks the silver of the negative, transforming it into silver sulphate, which dissolves in the solution. It must be used in an acid solution and is somewhat uncertain in its behaviour, occasionally refusing to act, and always acting more rapidly in measure as the reduction progresses.
Intensification.—Intensification is photographically the oppo site of reduction, the object being to increase contrast. This is done by the deposition of some other material on the silver image. A silver image, for instance, can be very much intensified by toning it with uranium, the reddish-brown uranium ferrocyanide having very great printing strength and converting a weak nega tive into one having a great effective contrast for printing pur poses. Usually, however, intensification is performed by deposit ing a silver, mercury, or chromium compound upon the image, and many photographic intensifiers depend upon the use of mer cury. Experience has shown that mercury intensified images are not usually as stable as images produced by chromium intensifi cation.
The mercuric intensifier consists of a solution of mercuric chloride, a few drops of ammonium chloride or hydrochloric acid being added to the solution. When the silver image is placed in this, it reacts with the mercuric chloride and forms a mixture of silver chloride and m,n-curous chloride. The bleached image, which appears white, can then be treated in various ways. If it is developed, for instance, both the silver chloride and the mercurous chloride will be reduced to the metal and to every part of silver there will be added an equal part of mercury. Instead of using a developer, the image may be blackened with ammonia, which forms a black mercury-ammonium complex and produces a high degree of intensification.
A very powerful method of intensification, used chiefly for negatives made by photo-engravers, is obtained by bleaching with mercuric chloride and blackening with silver dissolved in potas sium cyanide. The use of the cyanide cuts the shadows very slightly at the same time that the highlights are intensified, so that a great increase in the contrast of the negative is obtained. This is usually known as the Monckhoven intensifier.
In the case of the chromium intensifier the silver image is bleached with a solution of bichromate containing a very little hydrochloric acid, bichromate being an oxidizer of the same type as permanganate or ferricyanide. The image is then re-developed and will be found to be intensified by the deposition of a chromium compound. This intensifier has found increasing favour owing to the ease and certainty of its operation and the per manency of the intensified image.
Sensitometry.—The first quantitative measurements of the result of the action of light on photographic materials were made by Sir W. de W. (then Captain) Abney in 1874. (Abney, Phil. Mag., 48, pp. 161-165, 1874.) He constructed an instrument by which the transparency of the developed image could be meas ured, and by means of this he demonstrated that within limits the transparency of the deposit varied inversely as the logarithm of the exposure.