The evidence adduced from comparison is more or less certain according
to the skill of the expert and the circumstances of the case. Internal
evidence is such as is presented by the peculiar quality of lines when
drawn or worked up by slowly following traced lines, retouched shades,
rubbered surface of the paper, and every indication of an artificial
or mechanical process of producing writing.

Testimony based upon a knowledge of writing gained from having at some
time seen a person write is the most fallacious of all testimony
respecting handwriting; it can be only a mental comparison of writing
in question with such a vague idea or mental picture as may remain
from a casual view of the writing at some time more or less remote;
and besides, one may perceive another in the act of writing and yet
have little or no opportunity of forming any mental conception of it,
even at the time of writing.

In some cases where the courts will permit it the expert witness may
fully explain upon what he bases his opinion but it oftener occurs that
the trial judge will limit the evidence down to the very narrow scope
and the mere relation of such facts as the jury can see. Where a
forgery is well executed the difference in general appearance between
it and the genuine writing of the person whose signature is questioned,
when compared, is very small. The limit put upon expert evidence by the
trial judge takes from the effect of the testimony all the benefit of
an explanation of the facts upon which the opinion is founded.

Juries are generally allowed to examine enlarged photographs of the
writing, and sometimes to see it under the microscope, but even when
so doing what they see unexplained cannot be appreciated intelligently
and unless taken for granted as meaning something which the experience