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578 History of Three-Color Photography
parallax. F. Faupel answered this in saving that the thickness of the film should be at most one-half the width of the line. If the film contains twenty lines it will be 0.025 mm. thick. It is doubtful whether such sections can be made in celluloid. A cinematograph film should be at least 0.1 mm. thick, and if in such film the lines had a breadth of only 0.05 mm., there is bound to be great parallax. If a positive on an ordinary Krayn line screen be held up, the clear sky actually appears blue, but if it is viewed sideways it appears red or green.
F. Limmer13 found that the lines were about 0.042 mm. broad, and the distances in between were 0.11 mm., and there were thus 216 units to the square millimeter. W. Scheffer14 found that the thickness of the celluloid base was 0.013, the screen layer 0.005, the varnish 0.005 and the emulsion 0.020 mm. thick. Later a mosaic screen15 was prepared and the final form seems to have been prepared with a greasy resist (see p. 493) in which mordants were used.16
The Agfa Screen-Plate. — The pre-war plate was stated to be prepared with gum arabic, or other colloid. Von Hiibl17 stated that the polygonal grains had a diameter of 0.01 to 0.02 mm., and this layer was insulated by a celluloid varnish of about the same thickness.
The latest form of this plate, issued since the war, would appear to be different. H. Naumann18 stated that apparently it was made with gum arabic, or the like, stained and powdered and dusted on a tacky surface, and then rolled down. There are fine white lines between the grains but a filling is not necessary. The grains vary from 0.003 to 0.020 mm., there being about 7,000 per square millimeter. The color was stated to be about the same as the Autochrome, but the emulsion was faster, and the film more resistant to liquids and temperature changes. Several plates examined by the author had a very strong greenish tinge and quite unlike the Autochrome. W. Frerk19 stated that the color elements now appear to be shellac, and it was an improvement on the pre-war plate as it was more waterproof. The compensating filter is now orange, instead of yellow of the earlier one, and the green-sensitiveness of the plate is higher than the red.
The Paget Plate. — This it is believed was the invention of C. Finlay and consists of square elements, the blue being 0.063 mm. with 0.085 mm. for the red and green. The ratio of the squares was stated by J. H. Pledge20 to be roughly blue 8, green 7, red 7, there being two blue elements in the unit. The plate could be issued either as a separate screen, or coated with emulsion,21 though it is issued as a separate screen. Von Hiibl22 stated that the panchromatic plates, furnished by the Paget Plate Co., appear to be sensitized with isocol or similar mixture, and that the relative red, green, violet sensitiveness was very similar to that of the Autochrome plate, and that, therefore, a compensating filter of almost the same composition could be used. He specifically recommended :