a knowledge of which is requisite or of value in settling the point at issue. Persons professionally acquainted with the science or practice in question. Strickl. Ev. 408. Persons conversant with the sub- ject-matter on questions of science, skill. trade, and others of like kind. Best, Ev. § 346. The qualification of a witness as an expert is largely within the discretion of the trial judge; 61 Fed. Rep. 752; 132 Mass. 218; 63 Pa. 156: 126 id. 141; 108 N. Y. 61; 107 Ind. 84. Such a witness may be asked whether the examination made by him was superficial or otherwise; 158 U. S. 271, he need not be engaged in his profession, it is sufficient that he has studied it; 12 Ala. N. S. 648. Experts alone can give an opinion based on facts shown by others, assuming them to be true; 100 N. C. 457. "It is not sufficient to warrant the intro- duction of expert testimony that the wit- ness may know more of the subject of inquiry and may better understand and appreciate it than the jury; but to warrant its introduction, the subject of the inquiry must be one relating to some trade, pro- fession, science, or art in which persons instructed therein by study and experience may be supposed to have more skill and knowledge than jurors of average intelli- gence may generally be presumed to have;" 97 N. Y. 511; and not only may they testify to facts but they may give their opinions on them as experts; 118 N. Y. 429. The practical result of the rule admitting such testimony is far from satisfactory; its principal defect being that such witnesses are usually called because their known theories are understood to support the fact which the party calling them wishes to prove: 40 Cal. 405. "They come," says Lord Campbell, speaking of scientific wit- nesses, "with a bias on their minds to sup- port the cause in which they are embarked, and hardly any weight should be given to their evidence; " 10 Cl. & F. 154. In an- other case it was said that it was generally safer to take the judgments of unskilled jurors than the hired and biassed opinions of experts; 97 N. Y. 511; and such testi- mony is frequently characterized by the courts as of little value; 4 Dill. 488; 3 Bann. & A. 42; 6 Fish. 336; L. R. 6 Ch. Div. 415, n. See 32 Am. L. Reg. 529. On the other hand, the necessity of such testimony in certain classes of cases, par- ticularly those involving patent law, is thus set forth in 3 Rob. Pat. § 1012 :- "Notwithstanding the strictures passed upon ex- pert testimony by many jurists on each side of the Atlantic, and the truth of the assertions by which these censures have been justified, it is still certain that in most patent cases expert evidence is, and must always be, indispensable. That the expert is consulted before he is summoned as a witness; that when his opinion is unfavorable to the party who consults him he is not produced in court, at least on that side of the case; that when called as a witness his testimony is expected to support, and generally does support, the claims of the litigant on whose behalf he is presented, are no doubt true; but this is only what occurs in every other trial where counsel have properly prepared their case. The error lies with those who ascribe judicial functions to the patent-expert, and demand of him such free- dom from partisanship as the exercise of judicial powers requires. That there are experts in other departments of affairs upon whose opinion the court is forced to rely as the foundation of its own judg- ments, because incapable of forming an opinion for itself, and that such experts consequently fill the places of judges and should be beyond the influence and control of parties, must be conceded. But such the is not the case with patent-experts, whose opinion is received in evidence only in connection with th reasons on which it is based, and is to be accepted or rejected by the jury according to their own view of its fallacy or truth. The patent-expert, con- sidered in his reat character, is an explorer, gifted with unusual powers of discernment and apprehen- sion; a chronicler, trained to preserve the recollec- tion of the essential attributes of things: an ex- positor, fitted to embody those essential attributes in accurate and intelligible language; a monitor, able to suggest the conclusions which follow from the premises he has described. His relation to the jury is not unlike that which counsel sustain to the court, as guides to a correct decision of the issues severally confided to their judgments, the one pointing out facts and applying them in support of the claims advanced by his employer, as the other produces his authorities and applies them to the inaintenance of his claims of law." Such assistance, it is properly suggested, it would not be wise in any tribunal to undervalue or reject; 3 Rob. Pat. § 1012. The fact that the opinions of experts in patent cases are often diametrically oppo-