If fog has appeared on the plate developed in total darkness, we must try the effect of mixing a little fresh developing solution, being very careful to adhere to the instructions given. We remember a case of one amateur who had persistent fog, and eventually discovered that he had forgotten to put any bromide of ammonium into the No. 1 solution. Another discovered at last that he was using ounces of strong liquor ammonia instead of drachms ! If the developing solution is certainly correct, then the plates are at fault.
Colour Fog.
This includes red, brown, yellow, and green fog, all of which appear to be different degrees of the same disease, the first-mentioned being the most malignant variety, the last-mentioned the least so. It is unusual in the present day to meet with any but the green fog. When a negative showing the defect is examined by reflected light, a dark object being behind it, the shadows or most transparent parts appear of one or other of the three last-mentioned colours—brown, yellow, or green.
If the case be a bad one, on looking through the plate these shadows will appear of a red colour, the intensity of which may vary from ruby to a faint pinkness. If the defect be the slight green fog which may be met with under certain circumstances in the case of the best of plates, it is not visible by transmitted light, and is quite harmless.
In good plates colour fog makes its appearance only when insufficient exposure has been given, and excessive forcing with ammonia has been resorted to, and then particularly in hot weather. The remedy is to give ample exposure, or to use ferrous oxalate developer, with which latter the defect is never visible unless the chemi cals be impure.
In the early days of gelatine work frilling was one of the commonest of defects. The film of gelatine adhered so loosely to the glass that it frilled off when washing was being performed. The frilling generally commenced at the edge of the plate, and extended inwards, possibly only a quarter of an inch, possibly to the very centre of the plate. As the frilled portion stretched it could not be caused to lie flat on the plate, but, if the attempt were made, folded itself into wrinkles and spoiled the negative. At times frilling commenced in the centre, and the film rose in blisters.
At the present time frilling seldom makes its appear ance unless very soft water be used for washing. If
only soft water can be had, a few grains of Epsom salts should be added to each ounce of it and the alum bath should be used without fail.
Flatnes or Thinness of Image.
With good gelatine plates there is never any impossi bility in obtaining plenty of density or opacity, but it is quite likely that an error of judgment may be made, especially if the plate has been over-exposed, and the development may be stopped too soon. In this case it is necessary to intensify the plate,—that is to say, to increase the density of the image.
The following solutions are prepared : Water 1 Stuart.
Bichloride of mercury . 5 drachms.
Bromide of potassium . 5 drachms.
Water 1 quart.
1 Cyanide of potassium 5 drachms.
Nitrate of silver . 5 drachms.
After fixing, A is poured over the plate in a develop ing dish till the film appears to he whitened or bleached through its entire thickness. A very thorough washing follows, and then B is applied till the required density is got. Again the plate is thoroughly washed.
It is well, if a plate be found on printing to require intensification, to give it quarter of an hour or so in the fixing-bath first (afterwards washing). Intensification often reveals the fact that a plate, which appeared to be thoroughly fixed, is in fact not.
Too Great Density of Image is an error of the precisely opposite nature to that de scribed last. It occurs if the development be allowed to go too far.
A solution of thirty grains of ferric chloride in three ounces of water is prepared. The plate is placed in a fiat dish, and the solution is allowed to act on it for a short time. The plate is then washed and is placed in the fixing solution. It will now be found that the density of the image is considerably reduced. If it be not sufficiently so the operation may be repeated.
Spots.
Various kinds of spots appear in the finished negative at times. Opaque spots are due to defect in plates. Transparent spots with irregular outlines are also due to defect in the plates.
Perfectly circular transparent spots with well-defined outlines are due to bubbles in the developer adhering to the film; they are liable to occur if too little solution ho used. If the photographer be much troubled with this defect he should use a broad camel's-hair brush in development, sweeping it across the plate immediately after the solution is applied.