Joel Arunou, defense attorney in the Jean Harris murder trial, wasted no time today in pressing an attack on the credibility of one of the prosecution's top witnesses -- Westchester police detective Arthur Siciliano.

"Your statement wasn't intentionally meant to mislead, you simply forgot," Aurnou remarked sarcastically, his right thumb hooked into his vest pocket, after catching Siciliano in a contradiction early in the cross-examination.

Siciliano, a 30-year vetern with the Westchester police, has done little to mitigate the ambiguities of this trial, but he has produced testimony damaging to the defense -- some of which Arunou was trying to undo with today's cross-examination. He used every trick at his disposal in an attempt to trip up, ridicule and embarrass the witness. And when today's six-hour grilling was over, Aurnou apparently had succeeded in accomplishing at least a little of each.

Siciliano was one of the first police officers on the scene after the March 10 slaying of Dr. Herman Tarnower, a cardiologist and creator of the popular Scarsdale diet. The fatal shooting took place in the second-floor bedroom of Tarnower's fashionable home in Harrison, N.Y. He testified Monday that Harris, 57, former headmistress of the Maderia School in McLean, Va., told him she remembered "holding the gun and shooting him [Tarnower] in the hand." Tarnower died of four gunshot wounds inflicted by a .32-caliber revolver belonging to Harris.

Today Aurnou painstakingly went over Siciliano's previous testimony and written reports on the events of that night and the days that followed.

The first conflict concerned a bloodstained bedspread. Sicilian, under questioning by Aurnou, testified there had been such a stain on the spread over Tarnower's bed but none on the spread over the twin bed beside it. Then Aurnou produced a report the officer had signed shortly after the slaying, describing the stain the other way around. Siciliano was forced to admit the report entry had been "a mistake on my part."

Later Aurnou referred to earlier testimony on a bullet hole through the sliding glass door in Tarnower's dressing room. How high was the bullet hole? the lawyer wanted to know. Siciliano replied that it was 13 inches off the floor. Aurnou produced another report, signed by Siciliano, describing the bullet hole as being one foot, three inches off the floor. "Excuse me, excuse me," Siciliano apologized. "I just misinterpreted myself there. It was definitely 15 inches because I measured it myself."

"Didn't you also misrepresent yourself in yesterday's testimony?" asked Aurnou.

"Did I say 13 inches yesterday, too?"

"Don't you know?" Aurnou shot back.

"No, sir," Siciliano replied.

Aurnou kept after Siciliano on points of memory: What was the license plate number of Harris' car, which the officer had, on direct examination, bragged about memorizing? Did certain conversations with the Tarnower servants take place in person or over the telephone? What were specific markings on the murder weapon? Aurnou tripped up Siciliano on many of these items and, though several points were minor, the net effect was that of a witness rattled.

The jury and the defendant -- who faces a sentence of 25 years to life if convicted on the second-degree murder charge -- chuckled at an exchange between Aurnou and Siciliano toward the end of today's session, the 11th day of the trial.

Siciliano, after some hedging, admitted he had made an obscene gesture toward a couple of Aurnou's private investigators a few days after the slaying.

"Did you ever, in the course of this case, evidence bias, hostility, prejudice or profanity toward the defendant?" Aurnou asked. The witness denied doing so.

Then Aurnou brought up the incident with the investigators, who had gone to the Tarnower residence to deliver subpoenas. Siciliano acknowledged he had made a gesture as "a prank," but testified at first that he had simply raised his arm, "indicating they should leave." Then Aurnou produced a photograph.

"Weren't you just holding up the middle finger?" Aurnou prompted.

"It appears that way here," the policeman admitted. "I don't know if it was this finger or that finger," he added with a digital demonstration that provoked loud laughter in the courtroom.

Siciliano's cross-examination is scheduled to continue Wednesday.