muslin, as described under Emulsions (q.v.), into a solution of bichromate of potash, ten grains to the ounce, allowing it to stand for one hour, then washing for two hours in running water, or the addition of a few grains of cupric chloride or auric chloride will sometimes effect a cure. Chemical fog is often seen as iridescent green stains near the edges of plates, and is likely to make its appearance with an ammonia developer; it is less likely to ensue with the fixed alkalies, potash and soda, and rarely with ferrous oxalate. The obvious remedy for this evil, with a brand of plate known to be liable to green fog, is the use of potash and soda or ferrous oxalate ; but where it does exist it can sometimes be eliminated by treating the plate after fixing and washing with
Ferric chloride ... ... ... ... 50 grs.
Potassium bromide v. ... ... ... 30 ,,
Distilled water ............ 4 ozs.
Soak the plate in this for a minute or two, when it will be found that, as the fog disappears, the plate will be reduced in density ; rinse well, and then apply a ferrous-oxalate developer, when the required density can be obtained ; the plate should be then refixed and washed.
Light Fog makes its appearance generally all over the plate, and, as has been stated above, may be due to three causes, When the fault is supposed to lie in the camera (and the reason to suspect this is to find the edges of plate covered by rabbet of slide free from fog), cap the lens, remove the focussing screen, cover the head with the focussing cloth, and carefully examine the interior of camera to see if any stray ray of light is admitted. The plate may be fogged in the dark slide, and the effect will be seen on development by certain streaks and bands of fog making their appearance, a very usual place being at the leather hinge which allows the shutter of this slide to be folded back ; and this fog seems to be not only due to light, but in many cases is actually induced by the material or something used in making the hinge. , It may also make its appearance at the side where the manufacturer places the small slips of card-board to separate the plates, and in this case it is due to some impurity in the paper used. Stray actinic light or an unsafe light in the dark-room may also cause general veil or fog, and