Thus, by the transit
of this tracing point over any line upon the medal, the diamond
will draw upon the copper a section of the medal through that
line.
A screw is attached to the apparatus, so that if the medal be
raised a very small quantity by the screw, the copperplate will
be advanced by the same quantity, and thus a new line of section
may be drawn: and, by continuing this process, the series of
sectional lines on the copper produces the representation of the
medal on a plane: the outline and the form of the figure arising
from the sinuosities of the lines, and from their greater or less
proximity. The effect of this kind of engraving is very striking;
and in some specimens gives a high degree of apparent relief. It
has been practised on plate glass, and is then additionally
curious from the circumstance of the fine lines traced by the
diamond being invisible, except in certain lights.
From this description, it will have been seen that the
engraving on copper must be distorted; that is to say, that the
projection on the copper cannot be the same as that which arises
from a perpendicular projection of each point of the medal upon a
plane parallel to itself.
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