Thomas Gavie was convicted of possession of marijuana with intent to distribute, in violation of 21 U.S.C. § 841(a)(1). Numerous issues are presented on this appeal. Gavie contends that he was denied his constitutional right to a speedy trial, that the contraband found in his home and his automobile should have been suppressed, and that he was denied a fair trial because the trial judge 1 improperly commented upon the evidence and erroneously instructed the jury. Gavie further contends that the trial judge committed reversible error in- prohibiting the introduction of evidence concerning the polytypic nature of cannabis and overruling the defense’s motion to strike certain expert testimony concerning a laboratory test of the marijuana found in Gavic’s residence and automobile. We affirm the judgment of conviction.
The relevant portions of the record disclose that on November 28, 1973, a warrant authorizing the search of Gavic’s home was issued by the United States Magistrate on the basis of an affidavit signed by Agent John Boulger of the Drug Enforcement Administration. In his supporting affidavit, the agent stated that a reliable confidential informant, who had recently provided information concerning other controlled substances, had observed a large quantity of brownish tablets which one of the occupants of the house had described to her as THC (tetrahydrocannabinol).
Gavic’s home was searched pursuant to the warrant on the following day. There, special drug enforcement agents found LSD, amphetamines and a substance alleged to be marijuana. Gavie and his roommate, Gerald Seekon, were placed under arrest.2 Each was then asked for permission to search his automobile which was parked nearby. Gavie signed a “consent form” under circumstances sharply contested both in this appeal and before the court below. The subsequent search of Gavic's automobile yielded an additional seven pounds of suspected marijuana.
On the day of his arrest, Gavie appeared before a United States Magistrate; he waived a preliminary hearing and was released on his own recognizance. On July 2, 1974, some seven months later, an indictment was returned against him, charging him with *1349possession with intent to distribute approximately nine pounds of marijuana, seven pounds of which had been seized from his automobile and the balance from his residence. Gavie was arraigned on July 15, 1974, and following the denial of his pretrial motions to suppress the evidence seized from his residence and his car and to dismiss the indictment for failure to afford him a speedy trial, trial commenced on July 29, 1974. In accordance with a stipulation entered by the parties, the sole disputed issue presented to the jury was whether the substance seized from Gavic’s home and auto was, in fact, marijuana.
The government offered one expert witness who testified that the substance was indeed marijuana, and the defense offered an expert who testified that the tests employed by the government witness were too inconclusive to establish the presence of marijuana beyond reasonable doubt. The jury resolved the issue against the defendant.
I. SPEEDY TRIAL
Gavie first contends that the delay between his arrest, on the one hand, and his indictment and trial, on the other, was sufficiently long, unjustified and prejudicial to require dismissal of the indictment on Sixth Amendment grounds.3 Assuming, without deciding, that the eight-month delay between arrest and trial was sufficiently long to be “presumptively prejudicial,” Barker v. Wingo, 407 U.S. 514, 530, 92 S.Ct. 2182, 33 L.Ed.2d 101 (1972),4 we nonetheless determine that under the ad hoc balancing test announced in Barker v. Wingo, supra5 dismissal is not required here.
While it is true that the government has failed to offer an adequate reason for the delay in this case,6 this fact is offset by Gavic’s own failure to assert his right to a speedy trial. More importantly, he was not incarcerated during this period, and we discern no substantial prejudice to the defendant arising out of the delay. As the result of a pre-trial stipulation, moreover, the only witnesses at trial were experts who testified regarding the chemical properties of the substance seized by the arresting officers, thus minimizing any prejudice the delay may have engendered.
Conceding that the delay did not directly prejudice that phase of his case, Gavie nonetheless contends that he was seriously disadvantaged in his pre-trial motions for suppression of evidence because Gerald Seekon, who testified in his behalf, was unable to recall all the circumstances surrounding the challenged search of Gavic’s automobile. Based upon what recollection he had left, however, Seekon stated unequivocally at the suppression hearing that Gavie consented to a search of his automobile only after the agents threatened to tow his car *1350away and have it searched. Gavie himself gave similar testimony, adding that the agents threatened to “bust [the] trunk open,” while the prosecution elicited evidence from two narcotics agents that the consent had not been so coerced. In weighing such evidence, Judge Cooper credited the testimony of the agents rather than that of Gavie and Seekon. In so doing, Judge Cooper stressed not the gaps in Seekon’s testimony but rather Gavic’s obvious interest in the outcome of the case as well as his past criminal record. The trial court’s finding of lack of prejudice must stand unless clearly erroneous. United States v. Jackson, 504 F.2d 337, 341 (8th Cir. 1974), cert. denied, 420 U.S. 964, 95 S.Ct. 1356, 43 L.Ed.2d 442 (1975). We cannot say that these circumstances, standing alone, warrant “the severe remedy of dismissal of the indictment.” Barker v. Wingo, supra, 407 U.S. at 522, 92 S.Ct. at 2188.
II. THE SEARCH WARRANT
Gavie contends that the warrant pursuant to which his residence was searched was defective because the underlying affidavit contained hearsay and possibly unreliable information.
In the challenged affidavit, John Boulger, an agent of the Drug Enforcement Administration, recited that a reliable confidential informant had reported to him that she had observed at Gavic’s residence “a large number of brownish tablets which one of the occupants identified as THC.” The affidavit further noted that “[b]ecause of the difficulty of producing THC it is usual that other drugs are sold as THC,” i. e., LSD or phencyclidine. While acknowledging the statement in the affidavit that the same informant had recently provided other accurate information concerning controlled substances,7 Gavie asserts that, in questioning whether the substance observed was in fact THC, Boulger was impliedly questioning the informant’s reliability. Thus, he contends that the affidavit failed to satisfy the criteria set forth in Aguilar v. Texas, 378 U.S. 108, 84 S.Ct. 1509, 12 L.Ed.2d 723 (1964). We disagree.
Under Aguilar, two conditions must be met in order for an affidavit based on the statements of an undisclosed informant to support a search warrant:
* * * the magistrate must -be informed of some of the underlying circumstances from which the informant concluded that the narcotics were where he claimed they were, and some of the underlying circumstances from which the officer concluded that the informant, whose identity need not be disclosed, * * * was ‘credible’ or his information ‘reliable’.
378 U.S. at 114, 84 S.Ct. at 1514; see United States v. Marihart, 472 F.2d 809 (8th Cir. 1972) (en banc).
In this case, the informant’s statement that she had personally observed the tablets at defendant’s residence clearly satisfies the first of these requirements. See United States v. Harris, 403 U.S. 573, 578-79, 91 S.Ct. 2075, 29 L.Ed.2d 723 (1971) (opinion of Burger, C. J.). Moreover, Boulger’s unequivocal attestation to the informant’s reliability, note 7 supra, leaves no room for doubt that the additional comment was nothing more than it appears to be on its face— an observation that other contraband is often sold as THC. See United States v. Ventresca, 380 U.S. 102, 108, 85 S.Ct. 741, 746, 13 L.Ed.2d 684 (1965) (“affidavits for search warrants * * * mUst be tested and interpreted by magistrates and courts in a eommonsense and realistic fashion”). Aguilar’s second requirement is thus also satisfied.
Gavie contends in the alternative that, notwithstanding the reliability of the informant herself, her identification of the tablets as THC was predicat*1351ed on a statement of an occupant and, since nothing in the affidavit attested to the occupant’s own reliability, the accuracy of the information upon which the warrant was issued was open to question. In our view, however, a statement that there is a controlled substance on the premises by an occupant thereof is sufficiently adverse to his penal interest to create an inference of reliability.8 See United States v. Harris, supra, 403 U.S. at 583-85, 91 S.Ct. 2075; United States v. Carmichael, 489 F.2d 983, 986-87 (7th Cir. 1973) (en banc); Advisory Committee’s Notes to Rule 804(b)(3), Federal Rules of Evidence for United States Courts and Magistrates, Pub.L. No. 93-595 (Jan. 2, 1975). We are satisfied that the affidavit met the ultimate test: whether- it demonstrated adequate probable cause to support the issuance of a search warrant. United States v. Harris, supra; United States v. Marihart, supra; see United States v. Kleve, 465 F.2d 187 (8th Cir. 1972); United States v. Smith, 462 F.2d 456, 458-60 (8th Cir. 1972); United States v. DeCesaro, 502 F.2d 604 (7th Cir. 1974); United States v. Carmichael, supra; United States v. McCoy, 478 F.2d 176, 179 (10th Cir.), cert. denied, 414 U.S. 828, 94 S.Ct. 53, 38 L.Ed.2d 62 (1973).
III. CONSENT TO SEARCH THE AUTOMOBILE
We turn next to Gavic’s charge that his written consent to the officers’ search of his automobile was not freely and voluntarily given.
Both parties have briefed this issue as one governed by the test of voluntariness announced in Schneckloth v. Bustamonte, 412 U.S. 218, 93 S.Ct. 2041, 36 L.Ed.2d 854 (1973). While the holding in Bustamonte was narrowly tied to the noncustodial search in question there, 412 U.S. at 248, 93 S.Ct. 2041, we think the analysis therein has application to the custodial search before us in this appeal. See United States v. Campbell, 510 F.2d 967 (4th Cir. 1975); Hayes v. Cady, 500 F.2d 1212 (7th Cir. 1974), cert. denied, 419 U.S. 1058, 95 S.Ct. 642, 42 L.Ed.2d 655 (1975); United States v. Heimforth, 493 F.2d 970 (9th Cir.), cert. denied, 416 U.S. 908, 94 S.Ct. 1615, 40 L.Ed.2d 113 (1974). Bustamonte teaches that the voluntariness with which a consent to search is given “is a question of fact to be determined from all the circumstances * • * *.” 412 U.S. at 248-49, 93 S.Ct. at 2059.
On the record before us it is undisputed that prior to the search Gavie was given a form advising him of his rights, including his right not to consent to the search. It is also undisputed that Gavie signed the consent form after the agents had discovered in his residence various controlled substances of which he had admitted possession, had arrested him and had advised him that they would procure a warrant to search his automobile should he fail to consent to such a search. As we noted above, there is a conflict on the record regarding whether the agents also informed Gavie that his car would be towed and broken into if he withheld consent. The judge at the suppression hearing resolved this evidentiary conflict in favor of the government, emphasizing Gavic’s past criminal record and the self-serving nature of his testimony. Judge Cooper’s determination that the consent was voluntary under all of the circumstances, cf. United States v. Agosto, 502 F.2d 612 *1352(9th Cir. 1974), and cases cited therein, must stand as not clearly erroneous. See United States v. Watson, 459 F.2d 588 (8th Cir. 1972); Kilcrease v. United States, 457 F.2d 1328 (8th Cir. 1972).
IV. MONOTYPIC VERSUS POLYTYPIC NATURE OF CANNABIS
As the result of a stipulation entered prior to trial, the only issue for the jury was whether the evidence seized from Gavic’s home and his car was in fact a controlled substance, marijuana. The trial court rejected Gavic’s offer of proof that the genus “marijuana” is polytypic rather than monotypie, that in defining “controlled substances” Congress had outlawed only one species — Cannabis sativa L.9 — and that the government was unable to prove that the substance seized from him was Cannabis sativa L. In this appeal, he makes the same argument, assigning error to Judge Cooper’s refusal to permit the defense counsel to cross-examine the government’s expert witness regarding the polytypic nature of marijuana and to make an offer of proof of the legal authority in support of his defense.
Six other federal appellate courts have considered the issue raised by appellant. All have resolved this issue adversely to appellant. United States v. Walton, 514 F.2d 201 (D.C.Cir. 1975); United States v. Honneus, 508 F.2d 566 (1st Cir. 1974); United States v. Sifuentes, 504 F.2d 845 (4th Cir. 1974); United States v. Gaines, 489 F.2d 690 (5th Cir. 1974); United States v. Rothberg, 480 F.2d 534 (2d Cir.), cert. denied, 414 U.S. 856, 94 S.Ct. 159, 38 L.Ed.2d 106 (1973); United States v. Moore, 446 F.2d 448 (3d Cir. 1971), cert. denied, 406 U.S. 909, 92 S.Ct. 1617, 31 L.Ed.2d 820 (1972).10 Contra, United States v. Lewallen, 385 F.Supp. 1140 (W.D.Wis.1974). We find the reasoning of the First Circuit in United States v. Honneus, supra, particularly persuasive and instructive:
We are persuaded that Congress adopted ‘Cannabis sativa L.’ believing it to be the term that scientists used to embrace all marihuana-producing Cannabis; the other named sorts were not seen as separate Cannabis species. Linnaeus had listed but the one species, and appellant’s own expert concedes that until recently the monotypie view has been the ‘usually accepted’ one. This is not to try to refute Dr. Schultes’ [the expert’s] present view. The issue is not whether marihuana is monotypie or polytypic but what Congress meant when it used the term ‘Cannabis sativa L.’. We hold that the district court’s ruling and exclusion of Dr. Schultes’ testimony [concerning the polytypic nature of marijuana] were correct.
A remaining question is whether the definition was adequate to forewarn Honneus of the crimes for which he was later charged. We think it was. Right or wrong, the monotypie classification has had considerable currency until very recently at least; use of ‘Cannabis sativa L.’ was, we think, sufficient to put a procurer on notice that he acted at his peril, * * * and of course there is no evidence whatever that Honneus sought only ‘indica’ rather than ‘sativa’ or believed that he was acting legally.
508 F.2d at 575 [footnote omitted].
We adopt this reasoning, which controls on the facts before us. In view of our rejection of the legal theory on which Gavie premised this portion of his defense, we discern no prejudicial error *1353in the challenged evidentiary rulings below.
V. THE CHEMICAL TESTS
During cross-examination of the government’s expert witness, who had testified on direct concerning the results of chemical testing of the substance seized from Gavic’s home and car, defense counsel moved to strike all such testimony on the ground that the government had failed to comply with the “best evidence rule” because the test results themselves (thin layer chromatographic plates) had not been offered as evidence. The trial judge denied the motion to strike. Gavie assigns error to that ruling in this appeal, relying primarily on cases requiring a medical expert’s opinion testimony interpreting an x-ray to be accompanied by production of the .x-ray itself. Gay v. United States, 118 F.2d 160 (7th Cir. 1941); e. g., Fuller v. Lemmons, 434 P.2d 145 (Okla.1967).
In this circuit, we have repeatedly held that the admissibility of expert testimony lies within the sound discretion of the trial judge. See Gisriel v. Quinn-Moore Oil Corp., 517 F.2d 699 (8th Cir. 1975); Havenfield Corp. v. H & R Block, Inc., 509 F.2d 1263, 1272 (8th Cir. 1975); Mears v. Olin, No. 74-1565 (8th Cir., Feb. 28, 1975); United States v. McMillan, 508 F.2d 101 (8th Cir. 1974).
There was no abuse of that discretion here. It is conceded that Gavie did not move to strike the testimony in question until the government had completed its direct examination of the witness and cross-examination was well under way. In Chicago & N. W. Ry. v. Green, 164 F.2d 55, 63 (8th Cir. 1947), this court held that, where the defendant had failed to object promptly to secondary evidence of the contents of a document after such evidence had already been elicited twice at trial, it was within the trial court’s discretion to allow the testimony. See also Goetsch v. State, 45 Wis.2d 285, 172 N.W.2d 688 (1969) (where not requested by defendant, “best evidence rule” does not require prosecution to introduce original tests whose value lay wholly in expert witnesses’ interpretation thereof).
The Federal Rules of Evidence, though not in effect at the time of this trial, provide additional support for upholding Judge Cooper’s ruling.11 Rule 703 12 provides»
Bases of Opinion Testimony by Experts
The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to him at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence.
The test slides were highly perishable. Moreover, the defense expert did not dispute the readings but rather the reliability of such tests. We find no prejudicial error here.
VI. IMPARTIAL TRIAL
We have carefully scrutinized the record and reject appellant’s contention that he was denied a fair and impartial trial by the District Judge. The numerous allegations in support of this claim13 center upon the court’s instruc*1354tions to the jury. The trial judge summed up the evidence of the government and the defendant upon the single issue in the case: whether the substance seized from the defendant was in fact marijuana. When a federal judge undertakes to comment upon the evidence, as he has a right to do, he must present the evidence on both sides of the case and not concentrate on the strength of one side or the weakness of the other. In other words, he must be impartial. Gant v. United States, 506 F.2d 518 (8th Cir. 1974); United States v. Cole, 453 F.2d 902 (8th Cir.), cert. denied, 406 U.S. 922, 92 S.Ct. 1788, 32 L.Ed.2d 122 (1972); Scruggs v. United States, 450 F.2d 359 (8th Cir. 1971), cert. denied sub nom. Chambers v. United States, 405 U.S. 1071, 92 S.Ct. 1521, 31 L.Ed.2d 804 (1972); United States v. De Pugh, 434 F.2d 548 (8th Cir. 1970), cert. denied, 401 U.S. 978, 91 S.Ct. 1208, 28 L.Ed.2d 328 (1971); United States v. Liddy, 509 F.2d 428, 438 (D.C.Cir. 1974). Our reading of the instructions convinces as that this requirement was fully met.
Moreover, the District Judge repeatedly charged the jury that it was their own recollection of the evidence and their own evaluation of the expert witnesses which should guide them in their deliberations.14 See United States v. Williams, 505 F.2d 947, 949 (8th Cir. 1974), and cases cited therein. He cautioned the jury that the indictment was not evidence of guilt and instructed them in detail on the presumption of innocence and the government’s burden of proving guilt beyond reasonable doubt.
Appellant contends that at least twice during his charge the trial court advised the jury that possession of marijuana was conceded. A fair reading of those portions of the instructions is that the judge was speaking of the fact of possession, not the nature of the substance. He read the stipulation to the jury which made clear, moreover, that defendant only agreed that he believed the substance to be marijuana. Finally, he repeatedly reminded the jury that whether the substance was marijuana was the disputed issue in the case.15
*1355Appellant’s claim that the trial court assumed a partisan posture and diminished the effectiveness of the cross-examination of the government’s witness is likewise without merit. A trial judge is more than an umpire. United States v. Liddy, supra, 509 F.2d at 438. His duty is to see that truth is not withheld from the trier of fact, consistent with the rules of evidence. In this case the intervention of the trial judge in pursuit of clarity was neither partial to the government nor prejudicial to the defendant. We find no reversible error here.
Affirmed.