This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
The word halation signifies a "halo" or mist of light that surrounds and confuses the outline of an object. Halation is caused when some of the light that enters the camera passes through the film on the plate and is reflected from the back surface of the glass. Abney shows thata ray of light R (Fig. 1), passing through an emulsion containing particles of silver bromide P1. is not only reflected against P2, but, after passing through the glass at an angle, is again reflected to P3 as shown by the shaded portionsin Fig. 1. Halation may be prevented by coating the back of the plate with some preparation capable of absorbing light. A good anti-halation mixture is composed of caramel 1 part, burnt sienna 2 parts, gum 1 part, and alcohol 2 parts. This preparation is applied to the back of the plate with a brush. The plate may be conveniently held in a frame similar to that shown in Fig. 2. Cover the table with a sheet of clean blotting paper, and on this place the frame, which should be provided with carriers so that it may be used for any size of plate. Lay the plate, film side downwards, in the opening and rapidly brush over the back with the backing mixture. The backing, if properly prepared, dries rapidly. The backed plates should not be placed in the slides until the backing is thoroughly dry. otherwise dust will afterwards be found in the slide. Before developing the plate the backing should be rubbed off with a damp sponge. Most brands of plates may now be obtained ready backed. In taking interiors, dark trees against the sky, and, in fact, whenever strong contrasts are shown, backed plates must be used.
Halation in Negatives.