consequently became necessary to introduce a third colour and to secure a
special photographic relation between the three colours to prevent their
separation.

Photography, however, although the most dangerous weapon of the
counterfeiter, is not the only means of imitation available, a fact which
is sometimes overlooked. A note may be perfectly secure against
photographic reproduction, but from the absence of other necessary features
may be easily copied by an engraver of ordinary skill. There are two
systems of engraving employed in bank-notes:--(1) line-engraving in which
the lines are cut into the steel or copper plates; and (2) relief-engraving
in which the lines stand up above the plate as in wood-engraving. In the
former, adapted to the process called plate-printing, the ink is delivered
from the lines in the plate to the paper pressed upon it; in the latter,
adapted to surface-printing, the ink is spread upon the face of the lines
and printed as in typography. Plate-printing gives by far the finer and
sharper impression, but as there is a perceptible body of ink transferred
to the paper from the cut lines, it has been supposed that an impression
from plate would [v.03 p.0320] be more easily photographed than one from
surface where only a film of ink is spread upon the top of the raised
lines. But surface-printing being much less sharp and distinct than
plate-printing, imperfect copies of notes for which that process is used
are the more likely to escape detection. The plates upon which the early
notes were engraved being of copper quickly wore out and had to be
constantly replaced. The result was great difference in the appearance of
the notes, those printed from new plates being sharp and clear, while
others, printed from old plates, were pale and blurred. These differences
were a great assistance to the forger, as the public, being accustomed to
variations of appearance between different genuine notes, were less apt to
remark the difference between these and counterfeits.

In the early part of the 19th century, Jacob Perkins (1766-1849) introduced
into England from America what is known as the transfer-process, in which
the original engraving on steel is hardened and an impression taken from it
on a soft steel cylinder, which in its turn is hardened and pressed into a
soft printing-plate. By this means as many absolutely identical plates can
be produced as may be required, and being hardened they will yield a ve