OPINION
ROBERTS, Justice.At about 11:30 p. m., March 2, 1974, an altercation broke out between two rival gangs after members of one gang *322arrived at a party attended by members of the other. During the fighting which followed John Alicea received a fatal knife wound in the chest. Appellant, Whalen Laws, Jr., was charged with the murder. Following a jury trial, appellant was convicted of murder of the second degree. Post-verdict motions were denied, and appellant was sentenced to seven to twenty years imprisonment. In this appeal,1 appellant contends that the trial court, by its questioning of a witness and by its warning the witness of the consequences of perjury, improperly pressured the witness to change his testimony. We agree,2 reverse judgment of sentence and grant appellant a new trial.
I
During the presentation of the prosecution’s case, Coy Coley III was called as a witness. Unexpectedly, Coley gave testimony favorable to the defense. In his testimony, he indicated that the person who stabbed the victim was not in the courtroom.3 At appellant’s preliminary hearing Coley *323had identified appellant as the person who stabbed the victim.4
Over defense objection, the court questioned Coley in the presence of the jury about his testimony that he did not see the perpetrator in the courtroom.5 The court then excused *324the jury, and the prosecutor read Coley his testimony from the preliminary hearing. The court asked Coley if he recalled his prior testimony. Coley said he did. The court then asked Coley if he was afraid of anything on the day of trial, or if anything else was preventing him from telling the truth at trial. After Coley responded that there was nothing preventing him from telling the truth, the court, over defense objection,6 warned Coley of the consequences of perjury.7 The court then asked Coley if there was anything preventing him from telling the truth at trial, and asked Coley if any threats or promises had been made to induce him to testify untruthfully. Coley again answered no. The court asked Coley, twice, if he was telling the truth at trial. Coley answered both times that he was telling the truth. The court then asked Coley if he was saying that his testimony at the preliminary hearing was not truthful, and asked him if he wanted to read his testimony from the preliminary hearing again.
After Coley read his testimony from the preliminary hearing, the court again asked him if he was under any threats or pressure to testify as he did before the jury. Coley again answered no. The court then asked Coley, over defense objection, if what he told the jury was the truth and if he *325wanted to change that testimony now that he had read the notes of the preliminary hearing. Coley answered that he did not want to change his trial testimony. The court asked Coley, two more times, if he still maintained that his trial testimony was correct. After Coley answered affirmatively for the second time, the court asked Coley again if he wanted to change his trial testimony in any way. Coley said he did not. Before the jury was called back in, the court again warned Coley of the consequences of perjury.8
The Commonwealth then was allowed to cross-examine Coley in the presence of the jury, and the prosecutor read Coley his testimony from the preliminary hearing.9 Coley recanted his trial testimony and adopted his testimony from the preliminary hearing identifying appellant as the person who stabbed the victim.
II
In presiding over trial, the court has a paramount duty to maintain its impartiality, as well as a responsibility to respect the dignity of the witnesses. The court’s power to question a witness, and to warn the witness of the consequences of perjury, should be used sparingly, and with great circumspection.10 Efforts to clarify conflicting testimony *326should normally be left to counsel, and the resolution of any conflicts should be left to the jury. When the court intervenes in this process, it may unduly influence the witness to shape his testimony to what the witness believes the court expects. Cf. Commonwealth v. Myma, 278 Pa. 505, 508, 123 A. 486, 487 (1924) (“It is better to permit counsel to bring out the evidence and clear up disputed points on cross-examination unaided by the court . . . . The practice of a judge entering into the trial oí a case as an advocate is emphatically disapproved.”).
In Commonwealth v. Myma, Justice (later Chief Justice) Kephart stated for this Court:
“The judge occupies an exalted and dignified position; he is the one person to whom the jury, with rare exceptions, looks for guidance, and from whom the litigants expect absolute impartiality.”
Id. Thus, the court may not question a witness in a manner which conveys to the jury the court’s opinion on the merits, or doubts as to the credibility of the witness. Commonwealth v. Williams, 468 Pa. 453, 364 A.2d 281 (1976) (plurality opinion); Commonwealth v. Butler, 448 Pa. 128, 291 A.2d 89 (1972) (dictum); Commonwealth v. Myma, supra. The need to maintain impartiality also demands that the court exercise its authority with care, and refrain from questioning which may pressure a witness to testify in a particular way.
A review of the record convinces us that the trial court’s questioning of Coley and the use of perjury warnings unduly pressured Coley to retract the testimony he initially gave at trial, and to adopt his testimony from the preliminary hearing. The court’s questioning clearly conveyed the impression that the trial court did not believe Coley’s trial testimony, and that Coley should adopt his testimony from the preliminary hearing. The court asked Coley five times if he was telling the truth at trial. The court repeatedly *327asked Coley if any threats or promises caused him to testify falsely at trial, and asked Coley three times if he wanted to change or correct his testimony at trial. The court did not ask Coley if any threats or promises caused him to testify falsely at the preliminary hearing, or give Coley the opportunity to explain why he testified as he did at the preliminary hearing. The witness could have little doubt as to which testimony the court believed. The court’s questioning, coupled with the use of perjury warnings, improperly pressured the witness to change his trial testimony to conform with his testimony at the preliminary hearing.
The Commonwealth argues that the court’s action was harmless beyond a reasonable doubt. We cannot agree. Initially, Coley gave testimony favorable to appellant, indicating that someone other than appellant committed the killing. After extensive questioning by the court and admonishment on the consequences of perjury Coley retracted his earlier testimony and testified that he saw appellant stab the victim.
The Commonwealth relies on the testimony of two other witnesses. The first witness, Phillip Yant, testified that he saw appellant approach the victim. Although he did not see the stabbing, he did see the victim fall shortly thereafter. The second witness, Kevin White, testified that he saw appellant swing at the victim with a shiny object, after which the victim fell. White further testified that appellant later stated that he thought he stabbed the victim. According to White, appellant showed him the knife, and there was blood on the knife and on appellant’s hands.
We cannot find that the court’s action was harmless unless it was harmless beyond a reasonable doubt. See Commonwealth v. Davis, 455 Pa. 466, 317 A.2d 218 (1974); Salzburg, The Harm of Harmless Error, 59 Va.L.Rev. 988 (1973). The conclusion that the error was harmless beyond a reasonable doubt must be based on the court’s determination, beyond a reasonable doubt, that the error did not affect the jury verdict. Chapman v. California, 386 U.S. 18, 87 S.Ct. 824, 17 L.Ed.2d 705 (1967); see Commonwealth v. *328Lippert, 454 Pa. 381, 384, 311 A.2d 586, 588 (1973). The determination that the error did not affect the jury verdict may be reached “as a result of a finding that the impact of the challenged evidence is de minimis, or [that] the evidence supports a fact already established and ... no longer in dispute.” Commonwealth v. Rodgers, 472 Pa. 435, 460, 372 A.2d 771, 783 (1977) (concurring and dissenting opinion of Nix, J.); accord, Commonwealth v. Hale, 467 Pa. 293, 298, 356 A.2d 756, 758 (1976) (error cannot be harmless beyond a reasonable doubt if challenged evidence “was neither cumulative nor insignificant”).
Clearly, the impact of Coley’s change in testimony, viewed by itself, cannot be viewed as de minimis. The Commonwealth in effect argues Coley’s testimony is merely cumulative of that given by the two other witnesses.11
The circumstances are extremely limited in which this Court may find that damaging testimony — such as Coley’s *329testimony identifying appellant as the person who stabbed the victim — is harmless in light of other similar evidence. See generally Field, Assessing the Harmlessness of Federal Constitutional Error — A Process in Need of a Rationale, 125 U.Pa.L.Rev. 15 (1976). Professor Field has suggested that three requirements must be met before a court may conclude that improperly admitted evidence was merely cumulative of other evidence presented and therefore did not affect the jury verdict:
“(1) There should be substantial similarity, in type of evidence and incriminating factual details, between the tainted evidence and the untainted evidence of which it is ‘cumulative.’ (2) The untainted evidence should be indisputable, either because the facts are in some way affirmatively accepted by the defendant or for other reasons. (3) Care should be taken that the ‘untainted’ evidence in no way derives from the tainted evidence.”
Id. at 46.
We need not decide whether the testimony of Yant and White satisfies the first and third parts of this test, as it is clear that their testimony does not satisfy the second requirement: that their testimony be indisputable. The testimony of both witnesses was challenged on cross-examination, both as to ability to see the incident12 and as to credibility.13 Moreover, their testimony was contradicted by appellant and by appellant’s parents.14 In short, Coley’s *330testimony did not merely “support a fact already established and ... no longer in dispute.” Commonwealth v. Rodgers, 472 Pa. 435, 460, 372 A.2d 771, 783 (1977) (concurring and dissenting opinion of Nix, J.).
Moreover, the damage to appellant from Coley’s change in testimony arises not only from Coley’s adoption of his preliminary hearing testimony identifying appellant as the killer, but from his retraction of his earlier testimony at trial. Indeed, Coley’s testimony that the person who approached the victim immediately before the victim fell from the stab wound was not in the courtroom, if believed, would have strongly supported acquittal. The Commonwealth has not met its burden of proving that the trial court’s error did not affect the jury verdict.15
Judgment of sentence reversed and a new trial granted.
Former Chief Justice JONES did not participate in the decision of this case. *331NIX, J., filed a concurring opinion. EAGEN, C. J., concurs in the result. POMEROY, J., filed a dissenting opinion in which O’BRIEN, J., joins.. We hear this appeal pursuant to the Appellate Court Jurisdiction Act of 1970, Act of July 31, 1970, P.L. 673, art. II, § 202(1), 17 P.S. § 211.202(1) (Supp.1977).
. Because we reverse on this ground, we need not address appellant’s other claims that: 1) the court improperly denied appellant the right to call two witnesses to the stand; and 2) the court incorrectly summarized the evidence in its charge to the jury.
. Asked by the prosecutor if, on the night of the killing, he saw the person who stabbed the victim, Coley responded, “All I know is a light skinned person.” Coley, who had been a classmate of appellant’s and had known appellant for four years, stated that he did not know the name of the light skinned person. The prosecutor then asked if the light skinned person was in the courtroom. Coley replied, “No. I didn’t really see what happened.” In response to further questioning, Coley testified that he saw the light skinned person come close to the victim immediately before the victim fell from the stab wound, but that he did not see any contact between the two. The prosecutor again asked if the light skinned person was in the courtroom. Coley answered, “I don’t know.” Upon examination by the court, Coley clearly stated that the person he saw come towards the victim immediately before the victim fell from the stab wound was not in the courtroom. See note 5, infra.
. The transcript of the preliminary hearing included the following testimony by Coley:
“Q. Just what did you see happen?
A. John Alicia [sic] ran up to Whalen and Whalen stabbed him.
Q. Who stabbed him?
A. Whalen.
Q. Do you see the man in court who stabbed John Alicia [sic]?
A. Right there. (Indicating.)
Q. Indicating the defendant.”
. “BY THE COURT:
Q. Mr. Coley, you’ve testified that after the fight in front of the house where the party was going, groups went up towards 57th and Willows and something happened at 57th and Willows involving your friend John Alicea. Is that correct so far?
A. Right.
Q. You testified at that point a light skinned colored person went within touching distance of John Alicea. And then they all ran up the hill. And John Alicea was on the ground—
A. Right.
Q. —stabbed, is that correct?
A. That’s correct.
Q. Now, you were asked do you see the person you have so far described as a light skinned man that went within touching distance from John Alicea at that particular moment. You’ve given us an answer, first, ‘No,’ then you said, T don’t know.’
A. The answer is no.
Q. The answer is no to what?
A. To the question you asked me.
Q. I don’t understand.
MR. ELLIOTT [defense counsel]: I—
BY THE COURT:
Q. I asked you several questions. Now, what do you mean? No what?
MR. ELLIOTT: Objection, your Honor.
Q. You don’t know?
A. The answer is no, I don’t see the person.
Q. You don’t see the person—
A. No.
Q. —that you described as a light skinned colored man five feet seven, is that correct?
A. Right, that’s correct.
Q. Are you sure of that?
MR. ELLIOTT: I object.
THE COURT: Overruled.
*324BY THE COURT:
Q. Have you looked around the courtroom at everybody present in this courtroom?
(Brief pause.)
A. Yes, I did.
Q. And your answer is still you do not see that man in this courtroom, is that correct?
A. I do not see the man.”
. The defense entered, and the court noted, a continuing objection to the court’s questioning of the witness. Later .during the court’s questioning, the defense moved for a mistrial on the basis that the court’s action unduly pressured the witness to change his testimony.
. “BY THE COURT:
Q. Do you know what perjury is?
A. No.
Q. Perjury, generally speaking, means lying under oath. It’s a crime in the Commonwealth of Pennsylvania punishable by both or either prison or fine or both. Do you understand that?
A. Right.”
. “. . . And I want him [Coley] to understand — that’s the only reason I sent the jury out — a person who lies under oath is subjecting himself to perjury prosecution, which carries a substantially severe penalty, including prison—
MR. ELLIOTT: Your Honor, I object.
THE COURT: —and/or fines, if a person does lie under oath.”
. The prosecution may be allowed to cross-examine its own witness where the witness gives testimony at trial directly contradictory to statements the witness made earlier, and the witness’ in-court testimony is injurious to the prosecution’s case. Commonwealth v. Dancer, 452 Pa. 221, 305 A.2d 364 (1973).
. See ABA Project on Standards for Criminal Justice, Standards Relating to the Function of the Trial Judge § 6.4 (Approved Draft, 1972):
“The trial judge should be the exemplar of dignity and impartiality. . . When it becomes necessary during the trial for him to comment upon the conduct of witnesses ... he should do so in a firm, dignified and restrained manner, avoiding repartee, *326limiting his comments and rulings to what is reasonably required for the orderly progress of the trial, and refraining from unnecessary disparagement of persons or issues.”
. In Commonwealth v. Davis, 452 Pa. 171, 305 A.2d 715 (1973), this Court noted that United States Supreme Court cases allow an exception to the general rule that an error cannot be harmless unless the effect of the challenged evidence, viewed by itself, is de minimis. This exception is presented where the evidence is “so overwhelming,” and the challenged error “so insignificant” by comparison, that it is clear beyond a reasonable doubt that the error was harmless. We have cautioned, however:
“It should be emphasized that a conclusion that the properly admitted evidence is ‘so overwhelming,’ and the prejudicial effect of the . . . error ‘so insignificant’ by comparison, that it is clear beyond a reasonable doubt that the error is harmless, is not to be arrived at lightly.”
Id. at 178-79, 305 A.2d at 720; accord, Commonwealth v. Hale, 467 Pa. 293, 297, 356 A.2d 756, 758 (1976). Indeed, it has been suggested that an “overwhelming evidence” standard is inappropriate, and that in cases where the challenged evidence, viewed by itself, is not de minimis, it can be harmless only if it is merely cumulative of other, indisputable evidence. Field, Assessing the Harmlessness of Federal Constitutional Error — A Process in Need of a Rationale, 125 U.Pa.L. Rev. 15 (1976). In this case, however, we need not address the propriety of or any limitations on the “overwhelming evidence” standard. It is clear that the evidence in this case is not “overwhelming,” nor is Coley’s testimony “insignificant” by comparison. We cannot conclude that the court’s error was harmless unless Coley’s testimony was merely cumulative of other indisputable evidence.
. At the time of the stabbing it was dark, and both witnesses were occupied with other matters. Yant was fighting with another youth. White was running away, carrying an injured friend, while being chased by members of a rival gang. He testified he looked back and saw appellant swing at the victim. The record also indicates that another youth present during the fighting was similar in appearance to appellant.
. For example, White was a close friend of other youths, including one similar in appearance to appellant, who might have been blamed for the killing.
. Appellant testified that he was present at the time the fighting began, but went home before the victim was stabbed. His mother testified that she saw no blood, knife, or tom clothing on appellant when he arrived. Appellant, his mother, and his father all testified *330that after appellant arrived home, they saw the fighting from their front porch, and observed a youth, presumably the victim, fall.
. The Commonwealth relies on Commonwealth v. Moore, 462 Pa. 231, 340 A.2d 447 (1975). In Moore, this Court held that it was error to allow the Commonwealth to cross-examine one of its witnesses on the basis of an earlier statement by the witness identifying Moore as the person who shot the victim. Nevertheless, this Court held that the error was harmless error because two other witnesses identified Moore as the person who fired the fatal shots, and because the improperly cross-examined witness adhered to his initial trial testimony that he was unsure whether Moore or another person fired the shots.
Moore is not applicable to this case for two reasons. First, in Moore, the two other witnesses were “disinterested witnesses” who were in a position to clearly observe the events. Here, the testimony of Yant and White was challenged as to possible bias and as to their ability to observe the stabbing. Second, and more important, the witness improperly cross-examined in Moore continued to adhere to his initial testimony. Moore relied on Commonwealth v. Stafford, 450 Pa. 252, 299 A.2d 590 (1973), and Commonwealth v. Knudsen, 443 Pa. 412, 278 A.2d 881 (1971), which held that improper cross-examination by the Commonwealth on the basis of prior statements is harmless error if the witness adheres to his initial testimony. Under Stafford and Knudsen, the error in Moore would have been harmless even in the absence of other witnesses. In this case, however, the witness did change his testimony, and we cannot conclude that his change in testimony did not affect the verdict.