A good key to the psyche has always been that which promotes rage. There are barbs a person lets pass, barely noticing, and slights, however small, that make a person explode.

Jean Harris, in her murder trial here, has suffered any number of indignities in silence. The housekeeper of her lover, whom she is accused of murdering, testified that he tried to avoid her. Police, describing the night of the shooting, testified that she had told them her lover slept with "every woman he could."

All this she took quietly. But there was one bit of testimony Wednesday that she could not ignore -- the testimony from a police officer that in her response to Dr. Herman Tarnower's death the night of the shooting, she seemed to be "acting." That testimony -- a slur on the sincerity of her emotions -- enraged her. She yelled at one of her lawyers. ("All your clients talk to you like this?" his mother later asked.) She stormed out of the courtroom. She sought out reporters to talk. a

"I was honestly answering when I was asked those questions," she said.

So upset was she, in fact, that after her outburst -- during a recess -- a psychiatrist was summoned for a consultation. He was the same psychiatrist who had reportedly seen her after the Tarnower shooting last March in Harrison, N.Y., when she appeared distraught and depressed.

Today, however -- calm and composed -- Harris had a small victory. Judge Russell R. Leggett threw out Wednesday's testimony about whether Harris was "acting." It was not a judgment, he suggested to the jury, that could accurately be made.

"It's open to speculation," the judge said.

Former headmistress of the Madeira School in McLean, Va., Harris has been charged with second-degree murder in Tarnower's death. The prosecution, stressing the love triangle between Harris, 57; Tarnower, 69, and the much younger Lynne Tryforos, 37, claims Harris shot Tarnower in a jealous rage. The defense calls the shooting "a tragic accident," a suicide attempt inadvertently gone awry.

Today, as the trial went into its 13th day and the prosecution trotted out the latest in a series of police witnesses, there was little of the new or the dramatic. The police -- all members of the Harrison force -- for the most part repeated previous testimony. The defendant, in a simple black suit, took notes as usual, occasionally making a sardonic face and looking out toward the spectators as if for sympathy and vindication. An officer on the stand -- with the unlikely name of Harry Harrison of the Harrison police force -- might identify a bloodstained bra as the one Harris was wearing the night of the shooting. And Harris would flash her smile to the crowd -- "Can you believe this?" it seemed to say.

She smiled at testimony that centered around the shooting itself. She put on dark glasses during a line of questioning which her attorney, Joel Aurnou, asked a police officer if he had heard Harris express a wish to die. She also looked grim as one of the officers -- Patrolman Robertt Tamillo -- repeated testimony which had come out at the pretrial hearings, but which the jury heard for the first time today.

Under direct examination from Assistant District Attorney George Bolen, Tamillo recounted how, after arriving at the Tarnower home that evening and leaving the wounded doctor, he had gone to Harris, and helped her to the phone. It had been necessary to help her to the phone, the officer said, because Harris had appeared faint.

So, said Tamillo, he followed her to the phone, where he heard Harris' first words to a friend, who was also a lawyer.

"Oh, my God, I think I've killed, Hy," she reportedly said.

Additional witnesses for the prosecution are expected Friday.