President Reagan reached for one of the oldest, and least effective, laws on the books when he sought to chastise Democratic presidential candidate Jesse L. Jackson for his Cuban mission and to admonish him against trying to repeat the venture in the Soviet Union.

The president flourished the Logan Act at Jackson, suggesting that his personal diplomacy could be legally actionable. If the White House staff had done its homework, however, it would have known that the president was putting Jackson into select company by raising the ghost of a 185-year-old law that never has been successfully used to prosecute anyone.

Among those who have been threatened in the past with the Logan Act, sometimes described as "An Act to Prevent Usurpation of Executive Functions," have been Henry Ford, Sen. Joseph R. McCarthy (R-Wis.), Henry A. Wallace, Harold E. Stassen, William O. Douglas, Cyrus Eaton, Tom Hayden, Jane Fonda, Ramsey Clark and Pierre Salinger, not to mention George Logan himself, against whom the threat was so ineffective that he went on to become a U.S. senator in 1801.

In an interview taped for Orlando television stations on Monday, Reagan described the Logan Act as the "law of the land," but said he would not seek legal action against Jackson for his trip to Cuba.

In that the president was operating on pragmatic precedent, extending back to the days of President John Adams. According to legal scholars, only once did the United States attempt to invoke the Logan Act against anyone: a Kentucky farmer named Francis Fluornoy, in 1803, for writing an article urging the creation of a separate country in the American West, allied with France. That case was dropped when the United States made its Louisiana Purchase from France.

The law then was fresh on the books, inspired by congressional indignation over attempts by Logan, a wealthy Philadelphia Quaker physician, to negotiate with the French during the days of the Directory, when there was a threat of war between the United States and France. Bearing a letter from Vice President Thomas Jefferson, Logan interceded with French Foreign Minister Charles Maurice de Talleyrand to lift a French embargo on U.S. shipping and to free seized merchant vessels and imprisoned American seamen.

To the Federalist Congress, however, Logan's mission was treasonable, a Republican conspiracy involving Jefferson. In 1799 it enacted a law making it "a high misdemeanor" punishable by a fine of $5,000 and up to three years in prison (a whopping penalty in the 18th Century) for any unauthorized citizen to conduct any "correspondence or intercourse with any foreign government" related to "any disputes or controversies with the United States" or to "defeat the measures of the government of the United States . . . ."

Nevertheless, Pennsylvanians were so pleased with Logan that they sent him to the Senate.

In its current form, the prohibition on freelance diplomacy is Section 953, Chapter 45, of Title 18 of the U.S. Code. The language is regarded as so imprecise and of such disputed constitutionality, however, that some years ago the Justice Department recommended its repeal, as more precise statutes are on the books to punish explicit offenses in the field of foreign policy.

Among those threatened with prosecution under the Logan Act were Henry Ford, who was accused of violating it when he sailed his "Peace Ship" to Europe in 1915. Sen. Joseph McCarthy was threatened with the law by Secretary of State John Foster Dulles when McCarthy bargained with Greek shipowners to stop shipping to China and other Communist countries.

Dulles' predecessor, Dean Acheson, pointedly reminded Harold E. Stassen about the law when Stassen, then a private citizen, sent an open letter to Soviet Premier Josef Stalin in 1950. Earlier, in 1947, maverick Democrat Henry A. Wallace, a former vice president, created a furor when he denounced the "Truman Doctrine" during a tour of Europe.