THE SCREEN OE LIGHT FILTER.
Screens suitable for experimental work may be made very easily. Ordinary gela tine dry plates may have the silver salts cleared out, and the plate after washing immersed in the dye ; thin microscopic glass may be clipped in collodion, and after wards immersed in a suitable dye ; or plates may be coated with gelatine or col lodion, dried, and the films stripped off. As the last method is the most satisfac tory in use it will be described in prefer ence to the others, although slightly more troublesome. Take a sheet of patent glass, which should be close-ground and flat upon one side, the flattened side being readily detected. Rub this over with a waxing solution made by dissolving a little beeswax in turpentine, using sufficient of the latter to make a thin, easily managed medium, which may be applied readily with a small piece of flannel. The opera tion is the same as if preparing it to receive a double transfer carbon print.
Now coat it with 10 per cent. solution of gelatine, as if varnishing a negative. Care must he taken that the gelatine is properly in solution, and able to run easily. The measure and the glass must both be warmed before coating. When firmly set immerse the glass in a solution of aurantia until the desired effect is obtained. Allow to dry thoroughly, cut round the edges, and strip off the glass. Collodion may be used instead of gelatine, in the same manner. Some workers prefer to add the dye to the gelatine solution or collodion before applying to the glass. Such a screen should be a pale lemon yellow, and of sufficient depth to increase the ex posure by three times. It will be suitable for use in ordinary landscape work when necessary, where all that is required is to cut out the ultra-violet and slightly tone down the blue.