Bacon's multicolour printing-press, shown at the recent fair of the American Institute, will priut in 8 colours at a single impression. This is attained by a special arrangemeut of the inking-tablo, which, instead of being in a single piece, is composed of a number of narrow cast-iron plates held in a frame. These plates are formed of four distinct parts, and are wide in the centre and taper conically toward the extremities. This mode of construction allows them to move easily on each side at every revolution of the table, and has nearly the effect of an articulated joint. The end piece near the ink-trough is stationary. The various coloured inks aro placed in the ink-trough, which is dividi d into cells by metallic partitions. Directly over the trough, is an iron frame carrying a set of screws and nuts. By tightening these screws, which are placed over the metallic) partitions, the iuks as they flow beneath are prevented from mixing. The inking-rollers, instead of being fixed at a certain angle relative to the table, are arranged so as to run perfectly straight, the distribution being effected by the plates. The inks are spread on the multiple table in the usual way. As a eonsequence of the motion of the articulated joint, the inking-table is cau.sed to move slightly at every revolution of the table, and the ink is thus as well distributed as if several rollers were used. The movable plates which constitute the inking-table are of differeut widths, eo that the uppermost or tho lowest line in a page can bo printed in a colour selected beforehand. Motion is communicated to the movable plates by a small lever which hangs under the table, and which rests on a sruall vertical iron plate affixed to a cross-stay of the machine.
Chrome-lithography varies frorn simple colour work to tinted lithography and oleography. All coloured lithographs require a separate stone for each colour ; hence to ensure the correct position of each impression, it is necessary to have a "key-stone," on which the limits of each colour are distinctly and accurately laid down, the key-etone itself being omitted in tho printing, except for common work. The "set-off" or " faint," which is the " trace" produced by an iu3pression from the key-steue in au ordivary press (as described under Engraving on Steno), must have thu exact dimensions of the original, to ensure which, it is best to use good, stout, cream-wove note-papor, taking care that it is dry and well rolled, and the key-stone quite dry, and performing the operation without delay. The impression may bo dusted over with red chalk, and suapped with the finger nail to remove excess, aud may then be laid upon another stone, and passed through a press to communicate the imago. As to the order in which the colours should ho printed, this depends eomewhat upon the effect desired, but the general rules are that a dust-colour should always precede (not follow), and that transparent colours should succeed opaque ones, the common order being (1) dust-colours, (2) blues, (3) reds, (4) yellows, (5) outline- and finishing-coloure.
" Registering," or adjusting an already-printed sheet to the stone for further additions, is a delicate operation, which may be performed in three separate ways. The most simple is the " lay." The
paper should possess well-defined corners and edges, and its size is determined before making a set-off. This done, a " lay" corresponding to the edge of the paper is made on the stone, and the eubjeet is thus brought into position on the sheet, the lay being then drawn in fiue lines of litho graphic iuk, so as to print on the set-offs throughout. A set-off is made for each colour, and the lay,-mark is permanent. When printing a light colour, the lay-mark may be rendered permanent by oovering the place with gum, arid making seratalles through it when dry to coincide with the marks replaced, marking with common writing-ink, and washing the gum away when dry. The second method is by needles, taking advantage of permauent lines in the picture, or making tiny marks at the edges. The needles consist of slips of wood or cane in. lung and + in. thick, penetrated by a sewing-needle so that about 1 in. projects. The set-off on the stone is perforated by the needles at two opposite corners, and the sheet is pricked at the corresponding coreLrs. The needles are passed through the sheet from the back, and the sheet is thus dropped into its place on the stone, while the operator withdraws the needles and his fingers. This method is simple when the paper exeeeds the size of the stone. A third plan differs from the last only in having the needles fixed in a lath. These three methods (particularly the two first), though in very general use, proseut some objections, the removal of which is sought to be accomplished by two more recent plans. The first of these consists in letting leaden plugs about in. long and in. diameter into the stone for the reeeption of fixed wire pegs at the eeutre of each end of the stone, which are made to project about -eu in., and to puncture each set-off. When the stone much exceeds tho eize of the paper, one peg may be soldered to a strip of brass tinned at the back, and fixed to the sto.ne by blowing on a piece of shellac. The second improved method is to fasten two brass stri.ps, shaped respectively like the letters L, to the key-stone by means of shellac, and placing identical marks on each stone. Registering-machines have been devised, hut are not in general use..
The paper for colour-printing must be thoroughly stretched by rolling after it comes into the lithographer's hands. The temperature and degree of moisture of the room must be kept as constant as possible, to prevent stretching and shrinking in the paper. When printing on damp paper, it must be kept damp by covering it, and placing it out of draughts. The drying of the ink after each impression is best done in a special apartment at a sufficiently high temperature ; in this case, the paper should be similarly dried before making the first impression. The surface of the stone used is polished when pen or brush is to be employed, but grained for chalk and tints, the graining being coarser for colour than for black. The " setting-off " of colours may be avoided by dusting the printed sheets with a powder of chalk, talc, or magnesia, but this is liable to deaden the colours.