LEGAL EXPERTS may say it's not improper, but U.S. District Judge Thomas Penfield Jackson's public castigation of four jurors, sharp personal opinions on the guilt of Marion Barry and criticisms of two would-be visitors whom the judge initially excluded from the trial are extraordinarily imprudent -- and harmful to the very process he surely respects. Here was the judge who presided over this monumentally tense, important and, by its nature, nationally sensational trial, openly questioning the integrity and credibility of citizens drafted to serve as jurors and asserting that Mr. Barry was guilty of crimes far beyond the lone cocaine possession misdemeanor conviction that came out of the trial.

Our discomfort with the remarks of Judge Jackson does not stem from any direct quarrel with his conclusions. As it happens, we accepted the jury's verdicts as the workings of due process, and we did not find fault either with the sentence handed Mr. Barry by Judge Jackson or with the factors that the judge may have weighed in reaching his decision. What bothers us is the damage that these kinds of character attacks by a presiding judge on trial participants and observers, coupled with accusations of guilt beyond that found by a jury, may do to respect for the judicial process and in particular the jury system.

We do agree with specialists who say that none of this should supply Mr. Barry with any new ammunition in his expected appeal of his conviction. And certainly the judge's opinions are not grounds for any sort of sympathy votes next Tuesday to keep Mr. Barry in city government for another four years. Mr. Barry's own statements, as well as his conviction on the one charge, are ample grounds for concluding that the mayor's drug use was a terrible breach of the public trust. The disturbing postscripts of the presiding judge do not shake this conclusion, but neither do they serve any public good.