All evidence of handwriting, except where the witness has seen the writing in question written, is derived from four sources:  First, from comparison; second, from the internal evidence of the writing itself; third, from the knowledge of the writing, from having frequently seen a person write; fourth, where one has received letters whose authorship has been subsequently verified by admission, or acted upon in such manner as to receive the approval of the writer.  Comparison is made between the writing in question and other writing admitted by the writer to be genuine, or otherwise proved to be so to the satisfaction of the court.

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 of the expert who gives the opinion understands, and which they without such an education, could not be expected to understand that which the photographs show and the microscope makes visible is just as likely to be misleading as otherwise.

An expert may testify as to the characteristics of the handwriting in question; as to whether the writing is natural or feigned, or was or was not written at the same time, with the same pen and ink, and by the same person, and as to alterations or erasures therein; and as to the age of the writing and obscurities therein; the result of his examination of the writing under a magnifying glass; and to prove in some cases the standard of comparison.