The advance of photography : its history and modern applications (1911)

84 THE ADVANCE OF PHOTOGRAPHY the current is allowed to pass ; accordingly, plates of any thickness can be produced. If the original is incised, the electrotype impression will be in relief, and vice versa. Therefore, in the above case an impression is obtained with lines in high relief. This kind of plate is also adapted to give impressions, but rather differently from an incised copper plate. In the latter, the engraver's ink is rubbed into the incised marks, and then under strong pressure conveyed to paper. In a plate with a picture in relief, the impression takes place as in printing ; the raised places are rubbed over with printer's ink by the help of a leather ball or a cylinder blackened with ink, and then printed on paper. Letter-press is produced in this manner ; all its letters are in high relief ; also all wood-cuts which accompany the text. The printing-press is the simplest and cheapest mode of multiplying copies. It admits of the use of cheap papers, whilst copper-plate engraving requires a thick, soft, special paper. The printing-press, moreover, admits of woodcuts being printed in the text, whilst copperplate printing requires special plates. Lastly, the printing-press works with extreme rapidity, whereas copperplate printing requires much more time. Further, the printing-press does not use up the type rapidly, as it works under feeble pressure ; while copperplate printing, which requires strong pressure, wears the plate considerably, so that after striking off a thousand copies, the impressions are no longer as good as at first. The production of photographic plates for the printing-press is of the greatest importance, and Pretsch at first took the lead here. His process did not, indeed, produce the most perfect results. The incised plate which he produced on the gelatine film by the help of light was not deep enough to produce a high relief in the galvanic impression ; but this is necessary, for otherwise the