The Barnet Book of Photography: A Collection of Practical Articles. VariousЧитать онлайн книгу.

      Drying.—If the negatives are allowed to dry in the rack in which they were washed, the process is slow, and sometimes if the washing has not been complete, the middle portions of the negatives, which dry last, are less opaque than the rest. An excellent method of drying negatives rapidly and in such a way that no dust can fall on the film, is to drive nails (preferably of copper) into a wall or a board fixed against the wall, at distances apart depending on the size of the plates. Each plate then rests, with the film downwards, between a pair of nails, the lower corner of the plate resting against the wall, as shown on previous page.

      WINTER TIME ON THE ALPS.

       CAPT. W. DE W. ABNEY, C.B., F.R.S. Etc.

      Alum Bath.—It is frequently recommended that all plates should be immersed in a strong solution of alum, for the purpose of preventing "frilling" by hardening the film. Its use is, however, attended with the great disadvantage that liquids diffuse into and out of a film so treated with much greater difficulty than in the case of an ordinary film, and consequently if the film is alumed between development and fixing, the fixing is not only much slower, but the washing after fixing requires a very much longer time. If, therefore, the alum bath is used at all, it should not be applied until after the film has been well washed after fixing. The following solution may be used:—

      If a strong solution of alum is applied to the plate for a long time, the film may become so thoroughly hardened that it partially loses its adhesiveness, and there is a possibility that it will begin to peel from the glass after the negatives have been stored for some time in a dry place. The solution given above is quite strong enough and plates need not be immersed in it for more than five minutes, after which they must, of course, be again well washed. The hardening of the film, if not carried too far, no doubt makes it less liable to be injured by abrasion and the like.

      Frilling.—It sometimes happens that during the various operations of development, fixing and washing, the film begins to leave the plate and rise in puckers along the edges. This is known as "frilling," and in bad cases it may spread until a large part of the film has detached itself from the glass. It is due to excessive or irregular absorption of water by the gelatine, and at one time was commonly met with, but it rarely occurs with the dry plates of the present day. It is most likely to arise if there is any considerable difference of temperature between the various liquids and the wash-water, or during very hot weather when all the liquids are much warmer than usual.

      When frilling does occur, the plate must be treated carefully, so as to avoid tearing the film, but unless it is very bad and shows a tendency to spread, all the operations, including washing after fixing, should be completed before any special measures are taken to remedy the defect. On the other hand, if the frilling spreads rapidly, the plate should be carefully rinsed two or three times with water and placed for five minutes in the alum bath, with occasional gentle rocking, after which it is again well washed to remove the alum, and the various operations are completed. There is one exception to the procedure just indicated; if the frilling becomes bad while the plate is being fixed or during washing after fixing, the alum must not be applied until the fixing and the washing after fixing are completed. If something must be done in these circumstances, the plate, after draining, but without any previous washing, may be placed for about ten minutes in a saturated solution of common salt. It can afterwards be put back into the fixing bath, also without any intermediate washing, and the remainder of the process carried through.

      Although the methods just described will check the frilling, they will not remove its effects. For this purpose the plate after its final washing is allowed to drain thoroughly and is then immersed in methylated alcohol, preferably of the old kind, though the new kind can be made to do. The alcohol abstracts water from the film, which consequently shrinks to its original size and can be pressed back with the fingers into its proper position on the plate. Should the film be opalescent it should be removed from the first quantity of alcohol and placed in a second quantity, after which it should be set up to dry. The plates should not remain too long in the alcohol or the gelatine will contract too much.

      DEFECTS IN NEGATIVES.

      A perfect negative presupposes a perfect plate, correct exposure, and correct development stopped at exactly the right time. It is almost unnecessary to say that all these conditions are rarely satisfied, and consequently most negatives fall more or less short of perfection. The defects may be broadly grouped under two heads, namely, those due to imperfections existing in the film before exposure, and those due to defects or errors in the way in which the plate has been treated. It will be more convenient to deal with the latter, and larger, group first, but there is really no hard and fast division between them.

      The Negative is Thin, or in other words, whilst showing good gradation, and sufficient relative contrast between the different parts, is as a whole lacking in opacity or printing strength, and gives prints that are deficient in vigour and contrasts. The plate has been removed from the developer too soon, and the remedy is to intensify the image (see p. 51). Sometimes the want of opacity is due to the fact that the developer was too cold.

      The Negative is too Dense or Opaque and consequently although showing good contrasts and gradations, takes a long time to print, especially on dull days. The developer has been too energetic, or development has been continued too long; the remedy is to reduce the image (see p. 50).

      The Image is "Flat," or shows comparatively little contrast between the highest lights and the deepest shadows. This may, of course, be due to the absence of contrasts in the subject photographed; it is commonly due to over-exposure; it may be caused by using a developer containing too little reducer, or restrainer, or both, and too much alkali; sometimes it arises from a defect in the quality of the emulsion, or from the fact that the plate has been coated with an abnormally thin film of emulsion.

      The Image is "Hard," or shows excessive contrasts between lights and shadows, and is defective in the range of its half-tones. This is probably due to under-exposure, but may have been aggravated by the use of a developer containing too much bromide or too little alkali. Local reduction (see p. 50) may partially remedy the defect.

      Fog.—A more or less marked grey deposit of reduced silver extends over the whole surface of the image. It may be due to over-exposure, in which case the edges of the plate that have been protected by the rebate of the dark slide usually remain clear. It may also be caused by using a developer containing too much alkali, or too little restrainer, or both, or by the plate having been exposed to actinic light outside the camera, including the light from the dark-room lamp if the glass or coloured fabric used as the screening material is not efficient. In any of these cases the defect would be observable up to the extreme edges of the film.

      The character of the dark-room light should be tested by exposing one half of a plate to it at a distance of say nine or twelve inches for five or ten minutes, the other half of the plate being protected by some opaque substance. The best plan is to put the plate into a dark slide and draw out the shutter half-way. After exposure the plate is treated with a developer in the usual manner, and it can then be seen whether or no the light has exerted any action on the plate.

      Slight general fog may as a rule be neglected, but if the amount of fog is at all considerable the plate should be treated with a reducer,