FOCUSING SCREEN (Fr., Verre cloud, Glace doucie ; Ger., Visirscheibe, Mattscheibe, Mattglas, Mattglasscheibe) The screen upon which the image formed by the camera lens is focused, before exposing the plate, in order to secure sharp definition. It is usually of glass ground on one side to a matt surface. Some of the ground-glass screens supplied with the cheaper cameras are extremely coarse. A finely ground glass is, however, on the market in the usual cut sizes at very reason able prices. A good substitute, having the advantage of being light and unbreakable, is a sheet of matt celluloid ; but care must be taken that this does not buckle, or it will not agree in register with the dark - slide. For this reason celluloid is scarcely suitable for large cameras.
Temporary makeshifts, to replace a broken focusing screen, are : white tissue paper or tracing paper stretched taut, a fine cambric handkerchief, or plain glass dabbed lightly with putty. For Lohse's method of forming a focus ing screen the following is necessary: Gelatine . . 45 grs 450 g Barium chloride . 15 i 50 Ammonium sulphate. 7+ 75 Water to . . . 3+ oz. i,000 ccs.
The gelatine, sulphate and three-fourths of the water are heated together until dissolved ; the barium, dissolved in the remaining fourth of the water, is then added. After mixing and cooling, the mass is pressed through muslin so as to form threads, then washed and melted again. Finally a trace of salicylic acid in alcohol
is added, the whole is filtered, and is then ready for coating upon plain glass. The solution is slightly troublesome to prepare, but such a screen may with care last a lifetime. A less troublesome method, due to P. R. Salmon, is to apply to plain glass a varnish consisting of White lac . . . 70 grs. 8o g.
Picked gum sandarac . 12 ,, Alcohol . . . 2 OZ. 1,000 ccs.
C. Welborn Piper has suggested still another method ; a dry plate should be fogged uniformly all over by immersion in a developer for a long time, fixed, bleached in a solution of 5 grs. of iodine and ro grs. of potassium iodide in i oz. of water, treated with very dilute ammonia, washed, dried, and varnished. A screen pre pared in this way, or in any of the other ways above mentioned, is far superior to ground glass.
In process work, the ground-glass screen used has a transparent centre formed by cementing a thin microscopic cover glass to it with Canada balsam. Extremely fine focusing can then be done by means of an eyepiece. A cross should be made on the ground glass with a blacklead pencil before cementing down the glass, so that the focus of the eyepiece may be adjusted to it.