The mass of red hot
iron should be concealed from the observer's eye, both for the purpose
of rendering the eye fitter for observing the effect, and of removing
all doubt that the inscription is really read in the dark, that is,
without receiving any light, direct or reflected, from any other body.
If, in place of polishing the depressed parts, and roughening its raised
parts, we make the raised parts polished, and roughen the depressed
parts, the inscription will now be less luminous than the depressed
parts, and we shall still be able to read it, from its being as it were
written in black letters on a white ground. The first time I made this
experiment, without being aware of what would be the result, I used a
French shilling of Louis XV. and I was not a little surprised to observe
upon its surface in black letters the inscription BENEDICTUM SIT NOMEN
DEI.
The most surprising form of this experiment is when we use a coin from
which the inscription has been either wholly obliterated, or obliterated
in such a degree as to be illegible. When such a coin is laid upon the
red hot iron, the letters and figures become oxidated, and the film of
oxide radiating more powerfully than the rest of the coin will be more
luminous than the rest of the coin, and the illegible inscription may
be now distinctly read to the great surprise of the observer, who had
examined the blank surface of the coin previous to its being placed upon
the hot iron.
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