The Crossline Screen.—The Levy screen, now almost universally used, consists of two sheets of crystal glass, ruled diagonally with black lines by a patent process and cemented together with Canada balsam. The ruling in different screens may vary in fineness, from 5o to 300 lines per inch, according to requirements, and the relation of black to white may be either as t : t or 5 :4. Instead of being placed in contact with the sensitive plate, the screen is advanced a certain calculated distance in front of it, so that in place of the conspicuous pattern of the old-fashioned half-tone block, a series of dots appears of various sizes according to the gradations of the original. In practice, each aperture in the ruled screen acts as a pinhole lens, photographing a little picture, varying in shape and size according to the shape of the diaphragm aperture of the lens. A round aperture gives a round dot, a square aperture a square one, and so on. The object being to get several distinct kinds of dots, and especially to prevent vacant areas of transparency —above all in the high lights—the ordinary round lens diaphragm is of little service, as compared with the square shape. Various shaped diaphragms are employed in practice, such as lozenge shape, with extended corners ; or the Ray multiple diaphragms, pierced with three or more openings instead of one large opening, triangular, square, or round.
We have stated that the screens are ruled with varying degrees of fineness. Fifty lines to the inch is not too coarse for the daily newspaper tossed through the press at lightning speed, though some of the illustrated dailies print with 85, and even too line blocks in a very creditable manner. Fine blocks on art paper vary from 175 to 250, the former degree being sufficient for nearly all subjects. However, Mr. W. Gamble, the leading authority on photo-engraving, has pro duced blocks of still more delicate character, and is of opinion that, with the present high quality of letterpress printing, the 50o line block ought not to be regarded as an impossibility. In such blocks, the screen effect will have entirely disappeared, and the result be equivalent to the continuous tones of the silver print.
has rendered this fine degree of tone possible on metal has been the substitution of the fish-glue film for bitumen or albumen as the acid resist. Sixteen parts of fish glue and six parts of pure albumen are dissolved in the equivalent of their bulk of water, and sensitised with ammonium bichromate. The exposure of the metal plate does not usually exceed two minutes in sunlight, after which the superfluous glue is washed away, and the film is heated almost to charring point, when it forms an excellent resist. Copper is now employed in place of zinc, and the biting-in process is either performed with perchloride of iron, or Dutch mordant—the latter a very weak solution of hydrochloric acid, with a little chlorate of
potash.
improvements and variations are inevitable in the photo-engraving processes ; one that is worthy of record is the direct process of Mr. Arthur Payne, which is known as Paynetype, and does away with the necessity of a negative. The metal plates are to be pur chased ready for use, coated with a thin gelatino-bromide emulsion like ordinary dry plates, and are placed in the dark slide (behind the ruled screen if for half-tone work). After exposure the plate is developed in the dark room for 21 minutes with a glycin developer, rinsed under the tap, and then immersed in a 5 per cent. solution of potassium bichromate, which has the well-known effect of hardening the gelatine in the presence of the silver image. On the applica tion of hot water the soluble gelatine is washed away, as with a carbon print, leaving the negative image on the plate. Reversal is either obtained by rolling up the plate with litho ink mixed with varnish, or by the electro-deposi tion of a thin copper film on those portions of the zinc plate that are not protected by the image, after which the etching process may proceed in the usual manner, and the whole manipulations, as described, need not occupy more than about 15 minutes, a consideration, especially in newspaper work, apart from the saving of expense. For line work, and for all half-tone work up to the 85 or 120 screen, Paynetype possesses many salient advantages, and will greatly simplify the reproduction of up-to-date photographs in the daily press. When a negative is available the block can be made even more rapidly, because, as a positive is obtained from the negative, there is no need for reversal, and the original image on the zinc will form the basis of the resist.
Talbot-Klic process, a modification of that originally devised by Fox-Talbot, is not a very intricate one for the amateur with leisure and a little mechanical skill. A very carefully cleaned copper plate, so finely polished that not even the minutest scratch is dis cernible, and cleaned from grease by immersion, first in a solution of caustic soda, and then in very dilute nitric acid, is, when dry, prepared to receive an etching grain. For this purpose a dusting box is required, such as engravers use. Any closely made box will do, provided it is more than twice as large in area as the copper plates to be etched, and has a narrow, hinged door, or flap, extending along the bottom of one side. About a pound of very finely ground bitumen and colophony is put into the box, and the door closed, when it is shaken vigorously and turned upsidedown several times, until the atmosphere within is heavily charged with dust. Then the copper plate is inserted through the hinged flap, and the dust allowed to settle upon it for about five minutes.