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Color Filters or Color Screens 67
Allow 7 ccs. to every 100 qcm. The illumination through this screen is so bright that at a distance of 1 meter it is possible to read the text of this book, yet it is quite safe. For orthochromatic plates the transmission should be from 6300, and this can be obtained by combining one of the above screens with a violet one made as follows :
Crystal violet 0.166 g.
Cupric sulfate 0.833 g.
Glacial acetic acid 2 drops
Gelatin, 6 per cent sol 100 ccs.
Allow the same quantity per unit area as above. Half the above quantity of dyes gives a brighter screen, but it is not so safe. Another, which is equally satisfactory, is :
Tartrazin, 4 per cent sol 18.5 ccs.
Crystal violet 0.156 g.
Glacial acetic acid 2 drops
Gelatin, 8 per cent sol 100 ccs.
Allow the same quantity to the same area.
Liquid Filters. — Cells containing solutions have been frequently recommended ; but they possess no particular virtue, are always liable to leak, and the dyes may be precipitated on the walls. In those cases also, in which the bulb is directly immersed in the liquid, one has always to reckon with the heating of the solution and the slow evaporation of the solvent.
Howard Farmer31 made an exhaustive photometric examination of various glasses and fabrics and finally recommended a 6 per cent solution of potassium dichromate in a thickness of three-eighths of an inch. He stated that this was much safer than the majority of red glasses on the market, and yet gave brilliant illumination ; obviously the use of this, except for ordinary plates, would be attended with some danger of fog. E. Stenger32 also introduced a liquid cell of bell shape, which would hold 1000 ccs. of solution, as well as the lamp. For collodion plates, bromide paper and lantern slide work, he suggested a 10 per cent solution of potassium dichromate. For fast dry plates, the same solution with the addition of 5 g. fuchsin, or 0.2 g. acid violet 7BN, and for ortho and panchromatic plates 0.5 to 1.0 g. tartrazin with 0.2 to 0.4 g. methyl violet, or the above acid violet.
Green screens have also been used in liquid form and F. Haberkorn33 recommended the following:
Acid green, 2 per cent sol 25 ccs.
Naphthol green, 4 per cent sol 1.5 ccs.
Tartrazin, 3 per cent sol 1.2 ccs.
Water 800 ccs.
F. Novak34 found that with continued use the water very rapidly evaporated and, therefore, he suggested the replacement of the same with glycerol, and this was found not to lose in volume in months.