In his Aug. 17 report on the trial in Israel of John Demjanjuk, charged with crimes at the Nazi death camp Treblinka, Post reporter James Rupert cites the views of some observers that the three judges trying the case may be favoring the prosecution.

Having recently returned from the trial, where I spent two weeks observing Mr. Demjanjuk's testimony in his own defense and the prosecutors' lengthy cross-examination of him, I think there is an important perspective missing from such observations.

The judges did, at times, interrupt both prosecution and defense lawyers to put questions to Mr. Demjanjuk (as they have done with other witnesses for both sides), and this may seem "interventionist" to American observers accustomed to a more passive role by judges. But the judges' questions were objective and specific, reflecting the meticulous attention with which they are following the evidence, and I did not see any occasion where the judges' questioning unduly disrupted the orderly flow of examination that both prosecutor and defense counsel have a right to expect.

After Mr. Demjanjuk had been on the stand for several days, the judges began to caution him that they were finding certain answers evasive or incomplete. To an untrained observer, such expressions may well convey the sense that the judges are critical, and perhaps not impartial.

But trial lawyers know that judges are influenced by the demeanor of a witness and are continually forming impressions of the witness's credibility, whether they voice those impressions during the trial or not. In a jury trial a judge must be careful not to display any reaction that may influence the jury, but that circumspection is unnecessary where the judge alone will decide the case.

Were my client on the stand in a criminal trial, I would very much want to know when, and in what way, the judge found his testimony unclear, evasive or contradictory. By expressing their reactions openly, the judges are doing Mr. Demjanjuk's defense attorneys a service; at times, although in my opinion not often enough, Mr. Demjanjuk's attorneys took the cue and followed up with specific questions designed to repair the damage the judges had pointed out.

The three judges of the Demjanjuk trial, are, I think, quite conscious that the eyes of the world are on them and on the integrity of the Israeli judicial system, in a most sensitive case. Their "intervention" does not betray any prejudgment or partiality, but rather demonstrates a scrupulous determination to understand and weigh all the evidence, and especially the evidence that Mr. Demjanjuk himself offered in his own defense. ALLAN A. RYAN JR. Cambridge, Mass. The writer was director of the Office of Special Investigations, U.S. Department of Justice (1980-1983).