The 1978 Ethics in Government Act was passed in the wake of the Watergate scandal and amended in 1982. It requires the appointment of a special prosecutor whenever the attorney general determines after preliminary investigation that allegations of misconduct on the part of high-ranking officials are serious enough to warrant further investigation and possible prosecution.

The term "special prosecutor" was changed to "independent counsel" under the 1982 amendments to the act.

A ruling yesterday by the U.S. Court of Appeals struck down the judicial appointment of independent counsels, saying it violates the constitutional separation of powers doctrine. The ruling:

Overturns the legal authority of independent counsel Alexia Morrison.

Threatens the perjury conviction that independent counsel Whitney North Seymour Jr. won in the Michael K. Deaver case.

Does not affect independent counsels Lawrence E. Walsh and James C. McKay, who accepted Justice Department appointments last year to protect their investigations from constitutional challenges.

THE FOLLOWING PEOPLE HAVE HEADED PROBES UNDER THE ETHICS IN GOVERNMENT ACT:

Arthur H. Christy: Appointed in 1979 to investigate allegations that President Jimmy Carter's chief of staff, Hamilton Jordan, had used cocaine. Christy announced May 28, 1980, that a special grand jury had found insufficient evidence to prosecute Jordan.

Gerald J. Gallinghouse: Appointed in 1980 to investigate an allegation that Carter's national campaign manager, Tim Kraft, had used cocaine. Gallinghouse found no basis for prosecuting Kraft and concluded the investigation March 24, 1981.

Leon Silverman: Appointed in 1981 to investigate persistent allegations that Labor Secretary Raymond J. Donovan had violated federal law before joining the Reagan administration. Silverman reported June 28, 1982, that he had found "insufficient credible evidence" to charge Donovan with any federal crime. His inquiry was resumed in August 1982 because of fresh allegations, but ended in September 1982 with the same conclusion regarding Donovan's alleged ties to organized crime.

Jacob Stein: Appointed in 1984 to investigate questions raised about Edwin Meese III, the president's choice for attorney general. Stein reported Sept. 20, 1984, that his inquiry found "no basis" for bringing criminal charges against Meese.

Alexia Morrison: Appointed May 29, l986, to examine allegations that former assistant attorney general Theodore B. Olson gave false testimony to Congress in 1983 about the administration's withholding of Environmental Protection Agency documents. Morrison replaced James C. McKay, a trial lawyer who resigned from the investigation to avoid the appearance of an unspecificed conflict of interest.

Whitney North Seymour Jr.: Appointed in May, 1986, to investigate allegations of improper lobbying by former White House aide Deaver. Seymour was charged with examining whether Deaver violated federal conflict-of-interest laws in representing Canada, Puerto Rico and other clients after resigning as White House deputy chief of staff and setting up a consulting firm. On December 16, 1987, Deaver was found guilty of three of five counts of perjury and faces a possible 15-year prison term when he is sentenced Feb. 25.

James C. McKay: Appointed Feb. 2, 1987, to investigate former Reagan adviser Lyn Nofziger's lobbying on behalf of Wedtech Corp. and Attorney General Edwin Meese III's dealings with the company. On July 16, 1987, Nofziger was indicted on six counts of violating federal ethics laws and lobbying the administration on behalf of Wedtech, Fairchild Industries and a maritime union. His trial on conflict-of-interest charges opened Tuesday. On Dec. 22, 1987, McKay announced there was insufficient evidence to charge Meese with criminal activities.

Lawrence E. Walsh: Appointed Dec. 19, 1986, to investigate the Iran-contra affair. --------------------------------------------------------------------------

An unspecified number of other prosecutors, whose names have not been revealed, have been named to investig