DEFECTS IN THE NEGATIVE AND THEIR REMEDY Pinholes.—The annoying small transparent spots and micro scopic holes, often found in the negative, are generally to be ascribed to dust on the plate at the time of exposure. Plates should never be kept for more than a day or two in the dark slide ; somehow or other they will always accumulate dust there, and may possible absorb moisture from the air. The practice of wetting the plate with water before developing is another cause of these holes. Sometimes, but very rarely, they are due to defects in the gelatine. If the pinholes are very minute we must make up our minds philosophically to ignore them. Larger holes may be filled up with a mixture of India ink and gum water, with two drops of formaline. A very finely pointed brush must be used and the colour, almost dry, toned down to match the surroundings. The operation requires skill and exactness, and the colour tone must not be too dark or it will appear as white spots on the print.
Patches.—Irregular rounded patches of lighter tint, fre quently bounded by a fine dark line, are caused by the fact that the developer did not flow over the whole plate at once, and action commenced much later on these portions. Strangely enough, however long development may be pro tracted, these portions never "catch up" to the rest. Patches of varying density are due, as a rule, to parts of the negative drying before the rest, and at a different temperature. For instance, when half the film was dry, the plate may have been held near a fire to finish it off quickly.
Fog.—If the whole plate is masked by a grey fog and the margin covered by the rebate of a dark slide remains clear, we know that the fault is due to a leaky camera. A round halo in the very middle shows that the automatic shutter is slightly out of gear and lets in light while the camera closes. If the fog covers the plate, rebate edges included, either the developer contained an excess of alkali, or the dark-room light is defective. Streaks of light from the angles towards
the middle indicate that the joints of the dark slide are becoming loose, or that the slide does not fit close enough in the camera. Green fog is nowadays a rarity except with very stale plates, and will generally disappear in the clearing bath. Black spots of various sizes, some of them with curved tails, are caused by particles of developer insufficiently dissolved.
Powdery substance on the face of the film is due to impure chemicals giving a precipitate insoluble in water. Often this consists of a salt of lime from hard water used in washing the plate.
Intensification.—Very frequently our negative, although otherwise satisfactory, is lacking in density or contrast, and is so thin and weak that we cannot get good prints from it. It is possible in that case to deposit on the existing image a new one of chromium, copper, or silver. Before treatment the negative must be quite free from all traces of hypo (unless the chromium method is adopted) and if there is any surface fog it must first be removed by reduction (q.v.).
The old-fashioned and still widely practised mode of intensification by mercury is not one which is desirable. Apart from the deadly poisonous nature of mercury bichloride, the action of this substance on gelatine, the tendency of the negative to fade, and numerous other incidental uncertainties, all cry out against its adoption, now that safe and more efficient substitutes are available. In the mercury process the negative is first bleached white in a solution consisting of Mercury Bichloride . . . . . oz.
Hydrochloric Acid . . . to minims.
Water . . . . . . . . to oz.
the bleaching generally being fully accomplished within ten minutes. The plate is then washed in three or four changes of water and either redeveloped in any clean working developer, such as hydroquinone or rodinal ; or simply blackened by immersion in a bath of water 4 oz., ammonia i dram.