In Bamber's patent (3,252 of 1908) dyed gela tine is hardened with formaldehyde, dried, im mersed in water, heated to F. (100° C.), ground to powder, and sifted into varying sizes by elutriation with petroleum spirit ; then it is dusted on tacky glass and expanded by absorp tion of moisture from the air, the interspaces being then filled up with black filling. The average size of the particles is about viz in. (•or mm.).
The use of bichromated colloids was outlined by Du Hauron, and the first plate manufactured on these lines was the Warner-Powrie or Florence plate. The lines in this had a width of about in. (.04 mm.), and the green lines were formed first, then the red, and the interspaces were filled with blue. Another Warner-Powrie experimental screen plate had green lines with red and blue rectangles in between, thus follow ing more closely still the Du Hauron method; the diameter of the green line was in. (•05 mm.), the narrow diameter of the red areas being in. (.07 mm.) and that of the blue ch, in. (•05 mm.).
Dr. Smith, of Zurich, patented (6,881, 1906) a screen plate with regular geometric pattern, triangles, hexagons, or rhombi. The Thames plate (Finlay's patent, 19,652 of 1906) consists of a series of red and green circles of about in. (•ri mm.), with a blue filling. It is inter esting to note that in an American patent (561,687 of 1896) McDonough claims the use of ` a negative or positive photographic plate made with recurring patterns—as dots, lines, or figures—and having a portion where there is a fixed or invariable and distinctive charac teristic which is produced in the sensitive material of the plate, and by which the plate may there after be registered or adjusted in position in use." The Wratten patent (28,406 of 1907) has one distinctly novel feature in that the matrix, whether a linear or dot pattern, has the figures in black, semi-opaque, and clear, and therefore one would have, in printing on a bichromated dyed colloid, a quite insoluble, a half soluble, and a completely soluble film. After exposure, the insoluble gelatine would be stained deep blue, the half soluble only half that depth, and a clear line left ; by immersing such a plate in a yellow dye the half soluble line would absorb the yellow and give green, and the clear line could be subsequently coated with a red colloid and rendered insoluble. Practically this did not work out so well as it promised.
The Dufay dioptichrome plate (patents No. 11,698 of i9o8 and 18,744 of 1908) is prepared by exposing a bichromated colloid under a line screen, dyeing, and inking up with a greasy ink which does not adhere to the stained lines ; and this plate is now pressed against a gelatine-coated plate and the ink and dye are transferred. This second plate is coated with a varnish which does not adhere to the greasy ink, and then treated with a solvent which dissolves the ink and does not attack the coloured line ; and the plate then has one third of its surface red and two-thirds plain. A second printing at right
angles to the first gives the second colour, and then the unstained gelatine is dyed by pressure against another plate stained with the third colour. The green line has a diameter of in.
(.06 mm.), the red rectangle in. x in.
(.07 mm. x •I mm.) and the blue area in. x i-h in. (.065 mm. x •09 The only plates prepared by the fourth method so far have been experimental ones. Senor Cajal, of Madrid, suggested the use of coloured fibres embedded in celluloid rendered opaque by metallic silver and cut across with a micro tome. In 1893, Dr. Otto N. Witt applied for a German patent (W. 14,564, iv., 57A, Novem ber 1) for preparing linear filters by the super position of sheets of dyed celluloid, cementing them together and then rolling them out to thin veneers. On April 6, 1899, R. E. Liesegang applied for a similar patent, in which the block of superposed sheets of celluloid was to be cut across at right angles to the direction of the lines, and mosaic filters could be produced by cement ing such veneers into a block and again cutting across. R. Krayn took out two English patents (1,938 of 1906 and 495 of 1907) for precisely the same ideas, and experimental films of this char acter were issued.
The Krayn screen, which has been actually issued, belongs, like the Omnicolore plate, to the fifth method, in which mechanical printing is employed. The method is practically as fol lows : a greasy ink or waterproof varnish is ruled over two-thirds of the surface of a gelatine coated or celluloid film (Krayn), and the exposed gelatine is dyed and the water-repellent material removed and again applied so as this time to cover the dyed gelatine and half the unstained gelatine. The exposed gelatine is dyed and mordanted and then the remaining clear gela tine stained up. In the Omnicolore plate there are continuous blue lines of in. (•o5 mm.) width with green and red rectangles in between at right angles, the area of the former being about in. x in. (.o8 mm. x -o6 mm.), and that of the latter in. x -az- in. (.06 mm. x -04 mm.). In the Krayn celluloid film the lines are red and about -g-a cy in. wide, and the blue and green rectangles are not at right angles, but about an angle of x35 with the horizontal red line, and the space between the two red lines is about in. (.1 x mm.). Lumiere's patent (242,111 of 19o8) for a regular grain screen is based on the use of a greasy ink, which is applied to two-thirds of the surface of a gelatine film, the remaining third being dyed ; then the whole plate is varnished and the greasy ink and the overlying varnish dissolved. A second greasy ink is now applied in the form of lines at right angles to the first set, so that half the surface is covered, and the exposed gelatine is dyed in the second colour, again varnished, and the greasy ink removed ; and the plate, which now bears two colours, is dyed up as regards its remaining surface with the third colour.