FOCUSING SCREEN.— The material upon which the image formed by the lens is seen. This is usu ally a piece of ground glass of the best quality. It can be made by grinding a piece of patent plate glass with very fine emery pow der. The most convenient plan is to grind two pieces together by firmly securing one piece to a table and rubbing over it a paste of fine emery powder and water. The second piece of glass is then laid on and rubbed over the other in all directions. The upper piece of glass can be best manipulated by employing a pneumatic holder to lay hold of it. Another method of preparing glass is by treating it with hydrofluoric acid, which gives it a dull matt surface superior to ground glass. See Hy drofluoric Acid.
By oiling the ground glass the detail of the image is seen better. If the ground glass should happen to get broken, and another not easily obtainable, a rough temporary substitute can be made by coating a piece of glass with a paste of flour and water, or even a piece of tissue paper or fine cambric handkerchief can be used.
There have been many substitutes for ground glass recommended. A glass plate coated over with a matt varnish of— Sandarac t8 grains.
Mastic 4 Ether 200 mins.
Benzole roo " will make an excellent focusing screen, or even an ordinary varnish can be used if a little tartaric acid be added, and the varnish applied cold.
A paper focusing screen can easily be made with a piece of strong, but thin,paper, rendered translucent. (See Translucence.) Carey Lea coated glass with an emulsion of barium sulphate, made by dissolving thirty grains of gelatine in an ounce of water, and adding a small quantity of ammonium sulphate and a solution of barium chloride, stirring the while.
The side of the focusing screen next to the lens should be in . . . _ . .
exactly the same position that the film the dry plate or other sen sitive medium will occupy, otherwise unsharp images will be the result.
It is a good plan to mark the focusing screen, as shown in Fig. 199, with the different sizes
smaller than the camera is intended for, so that working with smaller plates the position of the view is plainly seen. The upright lines are also useful in detecting the verticality of architect ural subjects, etc.
very common defect in gelatine negatives. It appears as a veil covering the whole or part of the negative, and may be due to faulty emulsion, to the admis sion of stray light in the camera or elsewhere, or it may be due to over-exposure or to faulty development.
Chemical the error can be traced to the emulsion itself, it is thus termed. It usually makes its appearance as— Green appears green by reflected, and pink by transmitted light, being dichroic. It is produced by reduced metallic silver in a fine state of division, and also by the gelatine becoming decomposed by prolonged heating in the cooking process. The plates should be immersed in a strong solution of potassium dichromate, and then well washed. If any of the emulsion itself be left it should also be treated to the dichromate solu tion by squeezing it into the latter through coarse canvas, and afterwards well washing in water. It is as well to mention that this treatment will not always affect a cure, but it has been known to do so on many occasions, and is therefore well worth trying.
Plates liable to green fog should be developed with ferrous oxalate or a potash and soda developer, as there is less likelihood of it appearing with these developers than with ammonia.
Green fog may be eliminated from a plate by treating it after fixing and .washing to a bath composed of— Ferric chloride so grains.
Potassium bromide 3o " Distilled water 4 ounces.
or about two or three minutes. This serves to convert the image into silver bromide, and at the same time to bleach the green fog, and reduce the density of the negative. A ferrous oxalate developer is then applied, and the negative brought up to its required density, when it is refixed and washed.