site does not necessarily discredit their testimony but merely emphasizes the fact that their opinions are to be regarded as opinions, merely, and a decision rendered between them; Hall, J., in 4 Fish. 12. A patent expert is in effect an "auxiliary counsel" who argues upon the law and the facts; 28 Fed. Rep. 818. While expert evidence is not conclusive on the jury; 1 Fish. 17; and is to be judged by the same standards as ordinary evidence; 27 Fed. Rep. 691; 4 Fish. 404; 1 Sawyer 512;1 Fish. 298; and to be accorded by the jury such weight as they see fit; 1 Fish. 351; 6 McLean 303; 3 id. 432; it is nevertheless of great value in patent cases, 2 Fish. 465; 1 id. 133, 198, 461; 1 Bond 254; 5 McLean 44; 3 Story 742; 2 Robb 288; 3 McLean 432. The value of such testimony depends on the skill, not the number; 4 McLean 70; and is to be measured by their reasons; 3 Blatchf. 184; 4 Fish. 29, 232, 468. There are two classes of patent experts, as is clearly shown by an able writer, scientific and mechanical, each having a distinct sphere. The scientific expert is one familiarized by his studies and experi- ments with the principles of a science and qualified to understand, distinguish, and explain the subject-matter and application thereto of such science. His services are invoked to determine the character and scope of an invention with, reference to the condition of the art at the date of its production. His testimony is directed to the question whether the alleged invention is the result of an inventive act; whether it embraces or excludes a different inven- tion or is substantially the same in prin- ciple, function, or effect with any other. The mechanical expert represents the skilled workman in his art, who by practi- cal training in it could comprehend and apply to it various instrumentsand methods. His evidence will bear upon the defence of want of novelty, prior patent, inutility of the invention, or ambiguity of the descrip- tion in the specification of the patent. One person may appear in both capacities. 3 Rob. Pat. § 1013. See Curt. Pat. § 479. Expert testimony is admissible upon questions for the court as well as upon those for the jury, where it can be properly applied to the subject-matter of the ques- tion as the construction of the patent and whether a prior patent covers the same invention; 3 Rob. Pat. § 1014. In dealing with such questions the court is at liberty to admit expert evidence, but cannot be compelled to do so, and it is not error to refuse it; id.; 1 Fish. 487; 21 How. 88. It has been a matter of grave discussion whether an expert is bound to testify on matters of opinion without extra compensa- tion, the weight of decisions being that he is not bound to do so; 1 C. & K. 25; Sprague 276; 5 So. L. Rev. 793; 59 Ind. 15; 60 Ark. 508; contra, 6 Cent. L. J. 11; 21 D. C. 491; 59 Ind. 1, 15; 6 So. Law Rev. 706. It was recently held that, in the absence of statu- tory authority, one who testifies for the state in a criminal case as an expert cannot demand extra compensation as such, at least when not compelled to make any pre- liminary examination or preparation, or to attend and listen to the testimony; 60 Ark. 204. When no demand is made in advance for special compensation, an expert witness can recover oniy the statutory witness fees; 3 Colo. App. 177. See, generally, as to who are experts, and the admissibility of their evidence, 1 Greenl. Ev. 440; Tayl. Εν. 1209; 3 Dougl. 157; 2 Mood. & M. 75; 9 Conn. 55; 17 Pick. 497; 12 La. Ann. 183; 28 Am. L. Reg. 529, 593; 1 Am. L. Rev. 45; 5 id. 227, 428; 22 Alb. L. J. 365; 77 Cal. 579; 93 Mich. 511; 1 Misc. Rep. 354; 98 Ala. 285; 143 Ill. 571; 160 Mass. 131; Hershell, How to Use Experts; Etting, Exp. Ev. See also OPINION; PAT- ENT; HYPOTHETICAL QUESTION.