In a vivid and compelling bit of courtroom theater in the Jean Harris murder trial, an internationally known forensic expert donned the bloodstained pajamas worn by Dr. Herman Tarnower the night he was killed, and explained to the wide-eyed jurors his interpretation of the events of that night.

At the sight of the blue pajamas, which were nearly brown with gore, Jean Harris paled and averted her eyes. Tarnower's sister, who has daily been in court, had already left the room.

The scene was all the more dramatic because neither the defense, who had called this witness, nor the prosecution knew what the expert's testimony would be. He had examined the pajamas tops and a set of bloodstained bed linens only that afternoon.

The man's findings, however, seemed to strengthen the defense position -- particularly the contention that there had been a struggle between Harris and Tarnower the night he was shot.

The testimony also provided an outline of what might have occurred that strange March night when Jean Harris, according to the defense, "accidentally" shot her lover four times. The expert's findings seemed to indicate there had been time, on the night the doctor was shot, for someone to rest a bloody gun on the bed -- time, perhaps, even for someone to wash a wound. There was also testimony -- made on the doctor's interpretation of bloodstains on the sheets -- that indicated Tarnower had not been shot as he lay in bed.

"There was no evidence of high-velocity impact splatters on the bed," said the expert, Prof. Herbert MacDonell, after having explained to the jury several times that "high-velocity impact" results from gunshots.

There were, however, MacDonell continued, bloodstains that might have resulted from a high-velocity impact (i.e., shooting) some distance away from the bed. There were also bloodstains that suggested, as the defense likes to purport, that someone had slapped someone around.

"There were stains that could have resulted from a lower-velocity impact," MacDonell testified, "an impact from a slapping . . . a considerable amount of activity above and upon the surface of the bedsheet. . . ."

Former headmistress of the Madeira School in McLean, Va., Harris, 57, has been charged with second-degree murder in the shooting of her lover in the bedroom of his Westchester home last March. The defense claims the shooting was an accident, a suicide attempt gone awry. Tarnower, the defense lawyers say, was wounded in the hand as he wrested a gun away from Harris' head. The three other wounds, including the one to the doctor's back, they have yet to explain.

Instead, this week they have brought forth MacDonell, an $800-a-day expert on everything from fingerprints to ballistics to bloodstains, a sort of Forensics Decathalon Champ who has covered himself in glory in many a criminal trial.

In addition to his expertise, the droll, gray-bearded MacDonell has a certain charming eccentricity. He wears a wristwatch on both wrists because no one watch has sufficient charateristics to please him. He is never, as he goes about his business, without his pocket magnifying glass or his high-intensity flashlight. With him as he travels is a small entourage of adoring students, one of whom has brought his 13-day-old daughter to the courtroom.

But perhaps what is most significant about this gentleman is the brave new world of forensic science he represents. Spread your dirty linen before him and there is no end to the secrets he will expose. In the courtroom today, well aware of the dramatic impact of what he was about to do, he unfolded before the jury the lower sheet from Tarnower's bed. It was a flowered sheet, touched with yellow and pale blue, and had all about it the mean brown stains of blood.

It did not take an expert to see that the stains were blood or to recoil, instinctively, from the brown blotches on the pale flowers. But under MacDonell's interpretation, they ceased being so many bloodstains, and became records of specific events, as much as if they had been written in a language with a more conventional alphabet.

That spot there, the expert said, was obviously a "swipe" pattern, as when something which has been soaked in blood is moved across the surface. It should be distinguished from a "smear" pattern in which a pool of blood is simply, well, smeared. Those droplets, there, in the upper right of the sheet, are several droplets smaller than a normal drop of blood, they might be caused by arterial gush of a small artery. These patterns here -- most interesting -- they include stains of both fresh and partially clotted blood, and the good students on the jury will recall that blood takes at least 10 minutes to clot. Time elapsed somewhere here, a good quantity of time.

This stain on the side of the sheet, near what would have been the edge of the bed, is a distinctive pattern which clearly indicates the addition of water to blood, water -- he said -- causing blood cells to change in both chemistry and appearance. Was someone perhaps attempting first aid?

And that stain there -- with the distinct angularity of 90 degrees?

Place a small pistol upon it, note the stain below the trigger guard and under the hammer, and you will deduce that is exactly where someone would place a bloody thumb or finger, in an attempt to grab or pick up that gun.