155. Machine to produce engraving from medals. An instrument
was contrived, a long time ago, and is described in the Manuel de
Tourneur, by which copperplate engravings are produced from
medals and other objects in relief. The medal and the copper are
fixed on two sliding plates at right angles to each other, so
connected that, when the plate on which the medal is fixed is
raised vertically by a screw, the slide holding the copperplate
is advanced by an equal quantity in the horizontal direction. The
medal is fixed on the vertical slide with its face towards the
copperplate, and a little above it.
A bar, terminating at one end in a tracing point, and at the
other in a short arm, at right angles to the bar, and holding a
diamond point, is placed horizontally above the copper; so that
the tracing point shall touch the medal to which the bar is
perpendicular, and the diamond point shall touch the copperplate
to which the arm is perpendicular.
Under this arrangement, the bar being supposed to move
parallel to itself, and consequently to the copper, if the
tracing point pass over a flat part of the medal, the diamond
point will draw a straight line of equal length upon the copper;
but, if the tracing point pass over any projecting part of the
medal, the deviation from the straight line by the diamond point,
will be exactly equal to the elevation of the corresponding point
of the medal above the rest of the surface.
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