on a plate of metal, and on a block of wood, and let these two engravings be perfect fac-similies, line for line alike, in form, length, width and depth; then, the impression taken from the engraved plate of metal, being derived from the depressed lines, filled with ink, _into_ which the paper is forced by the action of the press, will present the appearance shown above. But, the impression taken from the engraved block of wood, being derived from the elevated portions of the block, covered with ink, _upon_ which the paper is impinged by the action of the press, will give the appearance presented below. [Illustration: =Printing=] Observe, therefore, that as on the two engravings, the same work produces results exactly the reverse, one of the other; so to produce the same effect from each of the two engravings, we must have recourse to two different processes. The former of the above two cuts, is the effect produced on paper from a metallic plate, into the surface of which the lines producing the shades are engraved or cut in. The same effects precisely may be produced on paper from a wood-block; but, in order to produce it, all the portions of the wood-block, which now give solid black upon the paper, must be cut out of the wood; leaving the lines, which now give white on the paper, prominent, so as to receive the ink and make their impression on the surface to be printed. The same end could be attained on the other side--that is to say, a light lettering on a dark ground--by cutting away all the metal, except the lines now producing dark impressions on a light ground, which would then give light lines on a dark ground; but the labor of doing this would be interminable, and the advantage gained, nothing. This principle once understood, the whole system becomes comprehensible at a glance. If, in an engraving on metal, all the lines cut into the plate were of equal depth and capacity, all the impressions would be equal as to shade, and the print would display an impression in pure black and pure white only, without intermediate tints. So, in cutting a wood block, if all the prominent parts be left equally prominent, the quantity of ink deposited by each and all will be identical, and the impression will be, as before, in simple black and white. To produce greater depth of shadow in one part of a metallic engraving than in others, the lines must be cut deepest where the shadow is to be the blackest, and thence graduated, less and less deep, to the plain surface, which gives pure white. To produce greater depth of shadows in one part of a wood-cut than in others, the prominent lines must be left most prominent where the shadow is to be the blackest; and thence shaved away more and more, as the shadows are to be less intense, until no lines at all are left on which the paper can impinge, and _there_ will be pure white.