THERE ARE ONLY a certain number of times you can say goodbye to Richard Nixon, and since, like everyone else in the country, we used up our quota long ago, we suppose it won't hurt to do it just one more time. The occasion, of course, is the apparent completion of the former President's series of four television interviews with David Frost.

In fact, Mr. Nixon, or whoever scheduled the interviews, seems to have saved the worst for last. There was Richard Nixon, whose severest critics even acknowledge his mastery of the elements of foreign policy, being senseless on the subject of Chile. And there was Richard Nixon, the one-time scourge of "permissiveness" in both our criminal-justice system and our way of thinking about criminal justice, wallowing in excuses for himself and Mr. Agnew and variously asserting and implying that neither man could have gotten a fair trial for his alleged crimes. Kingman Brewster, as President of Yale, once made a similar appraisal of a black revolutionary's chance of getting a fair trial in this country and the Nixon White House, under the rhetorical leadership of Mr. Agnew himself, pounced on the statement as evidence of a downright subversive attitude toward the American system of justice and practically everthing else.

On the subject of Chile, or more precisely the Nixon administration's relationship to the government of Salvador Allende and to the military coup that ousted it (killing Mr. Allende in the process), Mr. Nixon's general line of reasoning was aptly, if irreverently, characterized by Mr. Frost himself as "madness." The former President spoke of all of South America as a "red sandwich" between Castro's Cuba and Allende's Chile; he refused to make a distinction between the brutal unelected Chilean dictatorship of Augusto Pinochet and the elected, constitutional government of Allende. He seemed to regard the Allende government's economic failures and its suppressions of some civil liberties as being on a footing with the bloody acts of the soldiers who shot that government out of power.

Never mind that the CIA had reported that Allende was likely to lose the next election and that he did not intend to abolish democracy, Mr. Nixon said in response to a question - the CIA had been no damn good at predicting things anyway, and he, Richard Nixon, knew what the subtle and insidious Salvador Allende intended to do. You could tell that, he said, from reading the Allende elections speeches. The former President made a great deal of distinction between leftist governments, like that of Allende, that try to export their politics by subversion and force, and right-wing dictatorships, like that of Gen. Pinochet, that "are not enemies of the United States, and . . . do not threaten any of their neighbors." We found ourselves wondering after the program whether Mr. Nixon would ever notice the irony of his exalting nonintervention against one's neighbors as a critical value in the relationship between the United States and Chile.

But then there's really no point wondering about things like that. For if the interviews told us anything, they told us that Richard Nixon - self-pitying and self-justifying, willfully blind to the truths that may be just too painful to let in, and awash in a sense of the unfairness of it all - is no one from whom to expect perceptiveness or candor or, God helps us, an appreciation of ironies. In fact, the fourth interview was one from which we think people were only too eager to avert their gaze. It was nothing you could feel really bad about saying goodbye to.