We can see no valid objection to the competency of the evidence which the court permitted the witness So nth worth to give as an expert. The capacity of a witness to enlighten a jury on a matter of science or skill, or on a subject beyond the common experience of men, can be best estimated by the judge who presides at the trial of a cause ; and this court would be slow to revise a ruling on such a question, unless the error was very plain and palpable.
2. The rule, as now settled in this commonwealth, concerning the competency of evidence resulting from a comparison of a signature in dispute, with other signatures of the same party, requires that the handwriting used as a standard of comparison should be first established by direct proof of the signature, or other equivalent evidence. Commonwealth v. Eastman, 1 Cush. 217. Martin v. Maguire, 7 Gray, 178. This is necessary in order to avoid collateral issues, which would tend to distract the minds of jurors from the precise question before them. The *528same rule is applicable, for the same reason, when a signature not proved is . offered for the purpose of testing the accuracy of a witness. Until the jury have first settled whether the signature offered to a witness for his judgment is true or false, they can form no opinion as to his accuracy. If disputed signatures were admissible for the purpose of trying the correctness of the opinions of witnesses, a collateral inquiry would be raised concerning each signature so offered, and a case might be filled with issues aside from the real question before the jury. In the present case, the signature offered to the witness was neither proved nor admitted to be in the handwriting of the plaintiff, nor did the defendant propose to show that it was his genuine signature. The evidence was therefore rightly excluded.
3. The question as to the introduction of the plan, after all the evidence on both sides of the case had been put in, was entirely within the discretion of the judge who tried the case.
Exceptions overruled.