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Printing from Screen-Plates
541
E. Stenger and F. Leiber'1 dealt with this subject in a very exhaustive manner, and although they came to the same conclusions as to the reproduction of irregular screens as Mees and Piper, yet they pointed out the possibilities of using linear screens and explain very clearly why they maybe used. Their statements are theoretical, but were borne out in practice; but are unfortunately too voluminous to abstract, so that reference must be made to the original sources.
The makers of the Autochrome plate suggested4 an ingenious arrangement for copying these plates, which is as useful as any and very simple. A rectangular wooden box ABCD, Fig. 137, about sixteen inches in length, light-proof, the interior stained black to prevent reflection, is provided at one end AB with an opening which is fitted with a special filter E for giving correct effects with magnesium. This opening is covered by a movable slide V. At the opposite end of the box is a frame HI in which is placed the positive to be duplicated O with its glass side towards the interior of the box, then the unexposed plate, the glass side in contact with the film of the positive, then the usual black card. The frame is then closed by means of the shutter R.
Fig. 137. Lumiere Exposing Box.
A fixed support G carries a spiral of wire, fixed horizontally, in which is placed the magnesium ribbon M to be ignited ; the wire merely giving the ribbon the requisite rigidity. The support is fixed at a certain distance from the end of the box, and in such manner that the magnesium ribbon is level with the center of the filter. The diameter of the wire should be from 0.3 to 0.4 mm. and the wire so arranged as to give one ring of the spiral to each centimeter of ribbon. The magnesium ribbon is cut to the desired length, an average of 4 to 8 inches of ribbon one-eighth of an inch wide, according to the density of the picture to be copied, then folded in the center and placed inside the wire spiral in a vertical position. If, for example, 16 centimeters of ribbon be used, this when folded is reduced to eight, and will correspond to eight rings of the spiral. These details must be observed in order to insure regular combustion of the magnesium, the degree of rapidity of the latter influencing the faithfulness of the result.
The support G is fixed in such a way that the extremity of the magnesium ribbon is in front of the filter and exactly in the center of the box. The shutter V being closed, the magnesium is ignited by means of a