OMNICOLORE PLATE This is a screen colour plate of French manufacture, the patent of Ducos du Hauron and De Bercegorol. By means of a printing machine two lines in greasy ink are printed on gelatinised glass at right angles to one another, thus leaving little rectangles between the lines of gelatine, which is alone permeable to aqueous dye solutions. A compensating yellow screen or filter is used with the plates, and this and the filter elements reduce the speed of the emulsion to about 2+ Watkins, x.7 H. and D., or 27 Wynne.
It is advisable to manipulate the plates in the dark, and they must be placed with the glass towards the lens and a piece of opaque card in contact with the film to prevent it from being injured. The compensating filter should be placed behind the lens and the ground glass reversed, or with fixed-focus cameras the insertion of the screen behind the lens sufficiently lengthens the focus for ordinally purposes. The developer recommended is : Metol . . . 36 grs. 4 g.
Sodium sulphite (anhydrous) . . x oz. 5o Hydroquinone . . 18 grs. 2 lf Potass. carbonate (dry) If 30 Potassium bromine . 81 1 >, " Hypo " sol. (x : moo) 145 mins. 15 ccs.
Distilled water to . 20 OZ. 1,000 Development should be continued for five minutes, the plate washed for 15 to 20 seconds, and then immersed in the reversing bath of Potassium or sodium bichromate . . 70 grs. 8 g.
Sulphuric acid . . 114 111111S. 12 ccs.
Distilled water to . zo oz. 1,000 In which it should be left for about two minutes with gentle rocking. If a negative is required, naturally the plate is fixed after the first develop ment and not reversed. If a positive is required, then the plate in the bichromate bath should be taken out into daylight for three or four minutes, or for five or six minutes to artificial light, and as soon as the whole of the image is dissolved, immersed in a 5 per cent, solution of bisulphite lye or metabisulphite, or zo per cent. solution of sodium sulphite. The plate should then be re-immersed in the developer for five or six minutes, and when sufficiently dense, washed for about 3o seconds and fixed in Sodium hyposulphite 21 oz. 125 g.
Sodium metabisul phite . . . z65 grs. ' 25 Water to . . 20 OZ. 1,000 ccs.
Next wash for 20 to 3o minutes in running water, dry, and varnish with a 15 per cent. solution of mastic or dammar in benzole.
Under-exposed or too dense pictures may be improved by reducing in a "hypo" and ferri cyanide reducer; or weak pictures may be intensified with mercuric chloride followed by sulphite.