The Barnet Book of Photography: A Collection of Practical Articles. Various

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       Dock By Carine Cadby.

      Halation.—When the subject photographed includes some part much more brightly lighted than the rest, such as a window in an interior subject, the details of the bright part are not only lost, but the image of it seems to spread in all directions, obliterating the details of the surrounding portions. The effect is especially noticeable when the subject includes dark parts which necessitate a somewhat long exposure. A window at the end of a long dimly lighted interior, or dark trees against a bright sky are cases in point. The effect is really due to the fact that the sensitive film is not perfectly opaque, and some of the incident light passes through the film and is reflected from the back surface of the glass on to the under side of the film, producing a blurred image superposed, as it were, on the normal image formed at the surface of the film by the action of the direct light. The effect is known as "halation." It is prevented by having a perfectly opaque film, which is a condition difficult to realize in practice, and which, moreover, introduces certain other disadvantages. It is also prevented by coating the back of the plate with some substance that will absorb the rays that have passed through the film, and so prevent their being reflected back against the under side of the film. The substance used must either be opaque or must have a deep orange, brown, or red colour, and it must have the same refractive index as the glass, otherwise the reflection will not be prevented. For practical convenience it must also be easily applied and easily removed. Many substances have been recommended but nothing is so good as caramel, prepared by the action of heat on sugar. In order to get the mixture to dry completely after it has been applied, a somewhat troublesome process of purification is necessary, but caramel specially prepared for the purpose can now be obtained from dealers in photographic materials. The caramel (which is a solid substance) is dissolved in just enough water to make a thick syrup, which is carefully applied to the back of the plates in a thin layer by means of a flat brush.

      If the caramel does not dry properly the solution may be thoroughly mixed with about one quarter (or more) of its weight of very finely powdered burnt sienna or burnt umber, "ground in water."

      After being coated, the plates require some time to dry, and must, of course, be carefully protected from light. If the dark-room is thoroughly dark, the plates may be put up to dry in the same manner as negatives (see page 40), but if the dark-room is not suitable, some sort of drying box must be used.

      After exposure and before development the backing is removed with a damp sponge; if caramel only is used in a form completely soluble in water, it need not be removed unless a developer is being used that is to be applied to several plates in succession.

      REDUCTION.

When a negative is too opaque or dense it must be reduced by dissolving away part of the silver that forms the image. The same process is also applied for the removal of general fog, sometimes with a view to subsequent intensification.

      The simplest solution to use for this purpose is known as the Howard Farmer reducer and is a solution of hypo mixed with a small quantity of potassium ferricyanide (red prussiate of potash).

Ferricyanide Solution.
Potassium ferricyanide	1	oz. or	10	parts
Water to make up	10	oz. or	100	parts

      The negative which, if it has been previously dried, must be soaked in water for some time until it is thoroughly and uniformly wetted, is placed in some fresh hypo solution (the ordinary fixing-bath solution diluted with an equal volume of water) to which a small quantity of the ferricyanide solution has been added, and the dish is rocked repeatedly to ensure uniform action. The rapidity of the reducing action depends on the proportion of ferricyanide solution added, and it is very important not to add too much, otherwise the process gets out of control and reduction goes too far. The image should be carefully watched and the plate removed from the solution and rapidly washed before the apparent reduction is quite as great as it is intended to be. It is much better to stop too soon than too late, because if it is found that a little further reduction is necessary, the plate can be again immersed in the hypo and ferricyanide.

The ferricyanide reducer can be applied locally for reducing high-lights, halated windows, etc., and this is often very valuable, especially in the case of under-exposed negatives. A small quantity of hypo and ferricyanide solution is mixed in a measuring glass or some other suitable vessel. The plate is immersed in plain hypo solution in a white dish for a short time and is then raised by one corner or one edge until the part to be reduced is above the solution. The mixture of hypo and ferricyanide is carefully applied with a camel's hair brush to the parts that are too opaque, and after a few moments the plate is allowed to slip back into the hypo solution and the dish is rocked. If the reduction is not sufficient, the same proceeding is gone through as often as necessary. The reducer should not be allowed to act too long before putting the plate back into the hypo, otherwise the reduction may spread further than is desired. Further, the reducer must not be too strong (i.e., contain too much ferricyanide), otherwise it will produce brownish stains and the action may be too energetic.

      The other reducer is known as Belitzski's reducer, and is made up as follows:—

Ferric potassium oxalate	1	oz. or	5	parts5
Sodium sulphite	1	oz. or	4	parts
Oxalic acid	¼	oz. or	1	part
Hypo solution (25 in 100)	5	oz. or	25	parts
Water	20	oz. or	100	parts

[5] The formula in "parts" does not strictly correspond with that in ounces, but the difference is immaterial.

      The constituents must be dissolved in water in the order given. The solution can be used at once and it keeps fairly well if protected from light, in well corked bottles filled up to the neck.

      INTENSIFICATION.

      Intensification is a process in which the opacity of the image is increased by adding some fresh matter, metallic or otherwise, to the reduced silver that constitutes the developed image.

      The usual plan is to bleach the image by means of a solution of mercuric chloride (mercury perchloride or corrosive sublimate), which converts the dark-coloured silver into a white mixture of silver chloride and mercurous chloride, and this is subsequently treated with СКАЧАТЬ