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80 History of Three-Color Photography
C. Wolf-Czapek02 pointed out that there was always a chance that the filter might be forgotten, or that it might fall off, and proposed that the back of the screen-plate should be coated with a yellow-stained film, which would render any separate filter unnecessary. This method would be of special value with screen-plate films, as the stained coating would act as a non-curling coat.
A. Prilejaeff63 considered that aurantia extra MP (Agfa) was the best dye to use in conjunction with a little filter yellow, and gave the following formula :
Aurantia, 0.5 per cent sol 4 ccs.
Gelatin, 12 per cent sol 66 ccs.
The dye should be dissolved by heat and 7 ccs. of the dyed gelatin allowed for every 100 qcm. This filter was to be combined with one prepared with:
Filter yellow, 0.5 per cent sol 2 ccs.
Gelatin, 12 per cent sol 68 ccs.
The same quantity being allowed for the same area of glass, or the following could be used instead of the filter yellow:
Aesculin 0.66 g.
Ammonia 1 ccs.
Gelatin 8 g.
Distilled water 100 ccs.
Only 7 ccs. should be allowed for the same area.
All the above filters were intended for daylight work, but when artificial light is used, which differs in spectral composition in all cases from daylight, then the compensating filter should be altered, and the following were advised by von Hiibl04 : For Nernst lamps of 220 volts:
A. Gelatin, 10 per cent sol 40 ccs.
Tartrazin, 1 :2000 sol 3 ccs.
B. Aesculin 0.4 g.
Distilled water 37 ccs.
Ammonia 3 drops
Mix the above and add :
Gelatin, 10 per cent sol 40 ccs.
Patent blue, 1 :1000 sol 2 ccs.
Distilled water 38 ccs.
Allow 7 ccs. for 100 qcm. For incandescent gas the same solution may be used, allowing only 5 to 6 ccs. to the same area. For an arc lamp of 20 amperes:
A. Gelatin, 10 per cent sol 40 ccs.
Tartrazin, 1 :500 sol 4 ccs.
Phenosaf ranin, 1 :7000 sol 1 ccs.
B. Aesculin 0.4 g.
Distilled water 35 ccs.
Ammonia 3 drops