Kenneth T. Bainbridge, a distinguished physics professor who taught at Harvard for more than four decades, died Sunday in Lexington. He was 91.

Bainbridge, who was Leverett professor of physics emeritus, is perhaps best remembered for the work he did not at Harvard but on leave: directing the first test of an atomic bomb.

Recruited by J. Robert Oppenheimer to work on the Manhattan Project in Los Alamos, N.M. during World War II, Bainbridge was quickly put in charge of Project Trinity to prepare for the test, which ultimately took place on July 16, 1945.

"My personal nightmare was knowing that if the bomb didn't go off or hangfired, I, as head of the test, would have to go to the tower first and seek to find out what had gone wrong," Bainbridge wrote 30 years later in The Bulletin of the Atomic Scientists.

But the bomb test was indeed a success, ushering in the nuclear age.

As a researcher, Bainbridge was most noted for his work in measuring the masses of atomic nuclei. The professor designed and built powerful spectometers that determined mass with great accuracy, his colleagues said.

In addition, Bainbridge was instrumental in convincing the Navy to utilize radar on its ships to combat German submarines, recalls Edward M. Purcell, a retired professor of physics from Harvard.

"People now as a rule don't remember how chancy the thing was," Purcell says. "The war in the Atlantic was very nearly lost. The submarines were sinking American freighter ships by the dozens, I mean literally. It was radar that came along, and radar used on the ships finally took care of the submarine menace."

Bainbridge was also the first scientist to discover that the decay time of radioactive materials may depend on chemical composition.

Previously, most members of the scientific community had thought that the properties of nuclear disintegration were independent of outer environment.

Bainbridge's discovery placed him in direct head-on conflict with papal doctrine, recalls Mallinckrodt Professor of Physics Richard Wilson.

"The amusing thing is [Bainbridge's papers] came out just after one of these papal encyclicals, summarizing the state of scientific knowledge, which said that nuclei decay at the same rate, no matter the chemistry," Wilson says. "We were kidding him that he was in trouble with the pope."

Bainbridge was not ordinarily one to be embroiled in such controversy, though. He was described by those who knew him as quiet and dedicated to his science.

"A very fine individual--very thoughtful, very conscientious, very hard-working, almost meticulous," said Norman F. Ramsey, Higgins professor of physics emeritus.

Bainbridge was also cognizant of the dangers his discoveries could have caused, and he was conscientious enough to take action. After World War II, he actively opposed nuclear testing and the escalating arms race, and was one of 12 scientists who lobbied President Harry S. Truman to promise that the United States would not be the first country to use a hydrogen bomb.

The professor was also a staunch defender of academic freedom. As chair of Harvard's Physics Department in the 1950s, Bainbridge opposed the activities of Sen. Joseph McCarthy and the House Un-American Activities Committee.

As a teacher, Bainbridge was the author of several courses in the Harvard curriculum that incorporated new materials and new methods.

"Even in recent years when materials in experimental physics were changing very rapidly, Bainbridge was keeping up with it and designing courses around it," Purcell says.

Higgins Professor of Physics Sheldon L. Glashow, recalls a time when as a graduate student, he was taking an oral exam from Bainbridge, and was woefully under-prepared.

"All I recall is that he really wasn't very tough; he was [sympathetic]," says Glashow, who would go on to claim the Nobel Prize in physics. "He was a perfectly lovable gentlemen."

Bainbridge was born in 1904 in Cooperstown, N.Y., and spent most of his young life studying at Horace Mann, a prestigious New York school. He earned his bachelor's degree from MIT and his doctorate in physics from Princeton University.

After several post-doctoral fellowships, Bainbridge was hired by Harvard in 1934, and would not change schools until he retired.

He earned a promotion to associate professor in 1938. In 1946, after his war efforts had made him a national hero, he earned tenure from Harvard, and stayed on until retiring in 1975.