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Screen-Plat? Practice 515
the back or non-curling coating of a celluloid screen-plate, and if the original screen-plate was to be used as a positive by reversal in the usual way, the dye should be one that should be washed out or discharged in the operations of developing, etc.
The Focusing Screen. — As the sensitive surface of the screen-plate is turned away from the lens it is obvious that the matt surface of the focusing screen must also be reversed in the same way. If the thickness of the ground glass is exactly the same as that of the glass of the plate, there will be no trouble, but if it is not there may be slight diffusion with large aperture lenses. If, however, the image be focused with the filter in position, there is very little chance of this being of serious moment. If the filter be placed behind the lens after focusing, the ground glass need not be reversed, as in this position it automatically corrects the difference. The action of filters on the sharpness has already been dealt with under "Filters."
In order to avoid trouble and to enable the camera to be used for ordinary work as well, L. Schrambach24 placed on the market a screen, which had a diagonal of clear glass, and on one side of that the glass surface was matted on the inside and on the other on the outside. Exactly the same idea was utilized by H. Renezeder23 and a German patent26 was also granted for this. F. J. Hargreaves27 recommended, as a focusing screen, the use of an unexposed plate of the same make that is to be exposed. In this way the "grain" of the selecting screen was superposed, as it were, on the focused image, and the limitations imposed thereby were thus forced on one's attention whilst focusing. The bare screen will be found too transparent, and a coat of matt varnish or a very fine ground glass bound up in contact with the screen will be found an improvement. A better way was to fog an unused plate, develop, fix and bleach with mercuric chloride, as has been suggested for ordinary plates. A very dilute developer should be used and the plate fixed as soon as it is visibly darker by transmitted light. The whole operation can be done in daylight, if a dilute developer, such as rodinal, 1 : 100, be used. When the plate is dry it should be varnished, bound up with a cover glass, and mounted with its glass surface in register with the plate in the slide. As the plate thus used is one of those in actual use, absolute register is automatically obtained. It will be found that the colors appear slightly brighter on such a screen than on ground glass.
Exposure of the Plate. — As the emulsion coating of all screen-plates is extremely thin, it possesses but little latitude of exposure. The ill-effects of errors are more apparent than with ordinary black and white work, because one may have more or less complete falsification of the colors through such errors. The average observer is a far better judge of the correctness of colors than of a monochrome rendering of the same subject.
Numerous tables have been published by various workers28 but they