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Screen-Plate Practice 513
whirled for 30 seconds and dried in a calcium chloride box ; they were stated to keep in good condition for at least 10 days. The speed obtained was about half that by Monpillard's method. In a later communication11 Ninck laid stress on the importance of keeping the quantity of ammonia below 0.6 per cent, and suggested the following modification of the bath:
Pantochrome, 1 : 2,500 ale. sol 20 ccs.
Silver chloride solution 5.3 ccs.
Water 1000 ccs.
The increase of speed was found to be actually twenty-five times, and the plates kept for a month.
Xinck also stated14 that Autochrome plates, even 17 years old, might be utilized by hyper-sensitizing. It was preferable, however, to destroy the fog first by 5 minutes immersion in the following:
Chromic acid 5 g.
Potassium bromide 10 g.
Water 1000 ccs.
Then wash for 5 minutes and hyper-sensitize. Later15 he stated that the sensitiveness of the plates varied with the silver chloride content of the solution, the dye ratio remaining the same. Using an Eder-Hecht wedge, the speeds were, taking that of the Autochrome plate as unity, with pantochrome alone 6; with 0.07 g. silver chloride per liter, it was 17; with 0.18 g., 23; with 0.2 g., 33; with 0.53 g., 40. This last is the maximum quantity that can be used as with increase the speed drops, for instance with 1.6 g. it fell to 17. With the maximum quantity the plates must be used at once as they will not keep ; but it was possible to obtain at the beginning of April, with lens working at F 2.5, good results in 1/200 second.
Filling the Dark Slide. — As the light must pass through the screen elements before it reaches the emulsion, it is essential that the back of the plate be turned to the lens. Hence it is important that the glass be perfectly clean, otherwise any dirt or splash of emulsion will be reproduced as a patch of greater density in the positive, therefore, there will be less color. There is no difficulty in cleaning the backs of the plates, as after they have been placed in the slides, the glass can be readily polished with a clean dry cloth.
As the gelatin film of all screen-plates is extremely tender, great care must be exercised in placing them in the holders, and they are issued with a card in contact with the emulsion side, the purpose of which is to prevent damage to the film from the springs of the slide. This card is blackon the side in contact with the emulsion.
R. Chaboseau16 found that white card would do as well as black for backing up the plate, and although it did not reduce the time of exposure, it was without action in any way in producing fog or halation. L. Benoist17 stated that he had found that a white glazed card reduced the exposure from 20 to 30 per cent, and in a later communication he gave very careful
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