It would,
perhaps, be difficult to imagine the effects of some of these
kinds of engraving; but they would all possess, in common, the
property of being projections, by parallel lines, of the objects
represented, and the intensity of the shade of the ink would
either vary according to some function of the distance of the
point represented from some given plane, or it would be a little
modified by the distances from the same plane of a few of the
immediately contiguous points.
157. The system of shading maps by means of lines of equal
altitude above the sea bears some analogy to this mode of
representing medals, and if applied to them would produce a
different species of engraved resemblance. The projections on the
plane of the medal, of the section of an imaginary plane, placed
at successive distances above it, with the medal itself, would
produce a likeness of the figure on the medal, in which all the
inclined parts of it would be dark in proportion to their
inclination. Other species of engraving might be conceived by
substituting, instead of the imaginary plane, an imaginary sphere
or other solid, intersecting the figure in the medal.
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