The fact that the opinions of experts in patent cases are often diametrically opposite does not necessarily discredit their testi mony but merely emphasizes the fact that their opinions are to be regarded as opinions, merely, and a decision rendered between them ; Conover v. Roach, 4 Fish. 12, Fed. Cas. No. 3,125. A patent expert is in effect an "auxiliary counsel" who argues upon the law and the facts ; Steam-Gauge & Lantern Co. v. Mfg. Co., 28 Fed. 618.
The practice of introducing a large number of expert witnesses in patent causes is not to be commended, one competent witness on each side being usually sufficient to insure a full and fair elucidation of what is recondite in the case ; American Stove Co. v. Foundry Co., 158 Fed. 978, 86 C. C. A. 182. While expert evidence is not conclusive on the jury; v. Sizer, 1 Fish. 17, Fed. Cas. No. 9,056 ; and is to be judged by the same standards as or dinary evidence ; May v. Fond du Lac Coun ty, 27 Fed. 691; Carter v. Baker, 4 Fish. 404, Fed. Cas. No. 2,472, 1 Sawyer 512 ; Page v. Ferry, 1 Fish. 298, Fed. Cas. No. 10,662 ; and to be accorded by the jury such weight as they see fit ; Johnson v. Root, 1 Fish. 351, Fed. Cas. No. 7,411 ; Allen v. Hunter, 6 McLean 303, Fed. Cas. No. 225 ; Brooks v. Jenkins, 3 McLean 432, Fed. Cas. No. 1, 953 ; it is nevertheless of great value in patent cases ; French v. Rogers, 1 Fish. 133, Fed. Cas. No. 5,103 ; Carr v. Rice, 1 Fish. Q98, Fed. Cas. No. 2,440; Morris v. Barrett, 1 Fish. 461, Fed. Cas. No. 9,827 ; Parker v.
Stiles, 5 McLean 44, Fed. Cas. No. 10,749; Allen v. Blunt, 3 Sto. 742, Fed. Cas. No. 216; Brooks v. Jenkins, 3 McLean 432, Fed. Cas. No. 1,953.
It has been held that, without explanatory evidence, the defense of anticipation will not be considered in a patent case, where it is supported by prior patents for complicated machinery; Bell v. MacKinnon, 149 Fed. 205, 79 C. C. A. 163.
The value of such testimony depends on the skill, not the number ; Brooks v. Bick nell, 4 McLean 70, Fed. Cas. No. 1,946; and is to be measured by their reasons ; U. S. Annunciator v. Sanderson, 3 Blatchf. 184, Fed. •Cas. No. 16,790; Whipple v. Baldwin Mfg. Co., 4 Fish. 29, Fed. Cas. No. 17,514; Parham v. American Mfg. Co., 4 Fish. 468, Fed. Cas. No. 10,713.
There are said to be two classes of patent experts, scientific and mechanical, each hav ing a distinct sphere. The scientific expert is one familiarized by his studies and ex periments 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 in voked to determine the character and scope of an invention with reference to the condi tion 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 invention or is substan tially the same in principle, funCtion, or ef fect with any other. The mechanical expert represents the skilled workman in his art, who by practical training in it could compre hend and apply to it various instruments and methods. His evidence will bear upon the defence of want of novelty, prior patent, inutility of the invention, or ambiguity of the description in the specification of the patent. One person may appear in both ca pacities. 3 Rob. Pat. § 1013. See Curt. Pat. § 479.
Expert testimony is admissible upon ques tions for the court as well as upon those for the jury, where it can be properly applied to the subject-matter of the question as the construction of the patent and whether a prior patent covers the same invention ; 3 Rob. Pat. § 1014. In dealing with such ques tions 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.; Day v. Stellman, 1 Fish. 487, Fed. Cas. No. 3,690 ; Winans v. R. Co., 21 How. (U. S.) 88, 16 L. Ed. 68.
The opinions of experts are admissible to prove insanity ; U. S. v. Chisholm, 153 Fed. 808; to prove indebtedness by the general re sults shown by books of account; Brown v. U. S., 142 Fed. 1, 73 C. C. A. 187; to show whether a writing is genuine or disguised ; Rinker v. U. S., 151 Fed. 755, 81 C. C. A. 379; or whether a child had passed through the full period of gestation based upon the appearance of the child at the age of 13 months ; People v. Johnson, 70 Ill. App. 634; or the cause of a death from facts stated by other witnesses and without personal exami nation ; State v. Kammel, 23 S. D. 465, 122 N. W. 420 ; but such evidence is inadmissible to destroy the plain and obvious meaning of a contract where the words used are plain and unambiguous ; Bowers Dredging Co. v. U. U. S. 176, 29 Sup. Ct. 77, 53 L. Ed. 136 ; nor is it admissible upon the question of damages ; Lincoln v. R. Co., 23 Wend. (N. Y.) 425; Bain v. Cushman, 60 Vt. 343, 15 Atl. 171; Chandler v. Bush, 84 Ala. 102, 4 South. 207; nor as to whether they • were caused by negligence; East Tennessee, V. & G. R. v. Wright, 76 Ga. 532 ; International & G. N. Ry. Co. v. Kuehn, 2 Tex. Civ. App. 210, 21 S. W. 58 ; Hankins v. Watkins, 77 Hun 360, 28 N. Y. Supp. 867. See OPINION.