April 5, 1981

WESTBURY IT is normal for indicted politicians to express confidence that they will be vindicated. But Joseph M. Margiotta, the Nassau County Republican Party chairman, is acting like a man who is almost certain he will be exonerated. Mr. Margiotta has made almost a political and family outing of his trial on mail fraud and extortion charges in Federal District Court here. The 53-year-old Republican leader is charged with depriving Nassau County and the Town of Hempstead of honest government by presiding over a system in which $500,000 in municipal insurance commissions was dispensed over a 10-year period to politicians designated by Mr. Margiotta for little or no work. Mr. Margiotta was a somber figure - his face a mask of resignation -when the trial opened nine days ago with the selection of the jury. But his demeanor has changed since the testimony began last Monday. He frequently looks into the audience and smiles at friends and relatives from his position at the defense table, where he is flanked by as many as five lawyers.

STONY BROOK ''WHAT'LL it be tonight -turbot with fruit sauce or veal Parmesan?'' asked Arlene Nekrutman, the effeverscent manager of the Roth Cafeteria, one of the three main dormitory dining rooms at the State University at Stony Brook. The student, who wasn't familiar with turbot, a firm-fleshed white fish, was offered a taste. ''Not bad,'' he said, ''but I think I'll have the veal.'' For students attending colleges and universities on the Island, where and what to eat can be perplexing and often frustrating. As food costs skyrocket, attempting to satisfy students' palates and pocketbooks also presents problems to the purveyors. With more than 129,000 students on Long Island campuses, supplying several meals and snacks each day has become a big business. At Stony Brook, where more than one-third of the student body lives on campus - approximately 6,000 of 16,000 - more options of what, where and how to eat are available than at most other schools on the Island.

IN= the end, the familiarity of it all was the most dreadful. The shots, the shock, the screams, the Secret Service scramble to pack the stunned President into the limousine readying to rocket away, the affably serious doctors addressing anxious reporters in a makeshift hospital press room, the videotape over and over again that night, the columns in the next days on the when, why and how of American assassins - but above all, the fact that such a fact no longer astonishes. On Monday, March 30, at 2:25 P.M., John W. Hinckley Jr., a 25-year-old well-off dropout, tried to kill Ronald Reagan. The President had been in the Washington Hilton Hotel, selling his economic program to building trades unionists. Outside the narrow Presidential exit - built to Secret Service specifications - he turned to wave to the crowd, some press, mostly curious passers-by, that always gathers near a President. Mr. Hinckley fired six shots with a .22-caliber revolver loaded with explosive bullets called Devastators.

Consider the playwright cursed by the burden of memory, wounded by the pain of the past. Think, for example, of Eugene O'Neill. Old ghosts coursed his brain - scuttling up a synapse, running down a neuron, and hurling themselves onto the stage. Ah, misery! Oh, art! It is how we defined O'Neill. Now consider Neil Simon, who also dredges around his past. He exorcises old ghosts; sometimes he wrestles with demons, but for him they come out funny. For him they make us laugh. Him we define by the calcium deposit in his neck. It is an accretion built up by sitting hunched up over a typewriter, five hours a day for 30 years. Is this fair? Is this just? Where is the artistry and anguish in a calcium deposit? No matter; Neil Simon defines himself by the calcium deposit, too.

IN the language of statistics and econometrics, the attempt to assassinate President Reagan last week represented the movement of a ''random variable'' - a chance element that affects the economy in an unpredictable way but is unlikely to change its fundamental course. Random events are supposed to cancel each other out - unusually bad weather will be offset by a spell of unusually fine weather, the damage of an unexpected strike, like the current one in coal, by the eventual recovery from the strike, and the initial shock to confidence of the attack on the President by the gain in confidence from the recovery of the President. But are the shocks of the real world necessarily symmetrical and evanescent? From period to period, a random variable can assume an infinity of values, ranging from enormous to insignificant, from long-lived to momentary.