Paul Leon Hartman, a pioneering researcher and Cornell University professor emeritus recognized by his colleagues for his grace and humility, died at his home at Kendal at Ithaca on May 20. He was 91.

Hartman was one of the first to investigate the use of X-rays generated as a byproduct of high-energy electron accelerators. In the 1950s he and colleague Diran Tomboulian measured the spectrum of electromagnetic radiation from an electron beam circulating in a synchrotron, work that was background to the eventual use of high-energy X-ray radiation to study such things as the molecular structure of proteins. This work now goes on in the Cornell High Energy Synchrotron Source (CHESS).

"It was a gorgeous piece of physics," said Dale Corson, president emeritus, former chair of the physics department and close friend of Hartman's. "The spectrum had been calculated by somebody at Harvard, but Hartman and Tomboulian essentially confirmed the calculation. It really was a tour de force."

Hartman was also a writer, historian and amateur astronomer, as well as an outdoorsman and an expert baker. In his 71 years of affiliation with Cornell, he was known as an enthusiastic and dedicated teacher with a quiet habit of noting his colleagues' achievements while downplaying his own.

Hartman was born July 13, 1913, in Reno, Nev., the oldest son of Leon W. and Edith K. Hartman. He earned a B.S. degree in electrical engineering at the University of Nevada in 1934, then worked briefly in the produce department of a Safeway store. He recounted the experience in 1984 in his characteristic wry prose: "The district manager later told me I had a bright future in the discipline," he wrote. "He wasn't sure about physics."

Nonetheless, Hartman left the produce department for Cornell later that year and received his Ph.D. in physics in 1938. During World War II, he helped develop radar centimeter-wave generators for Bell Telephone Laboratories in New York City. He returned to Ithaca in 1946 to join the Cornell faculty as a charter member of the Department of Engineering Physics (now the School of Applied and Engineering Physics) with a joint appointment in the physics department.

Along with his work on synchrotron radiation, Hartman studied far ultraviolet radiation, optics and the solid state physics. He spent three sabbatical leaves and many summers at Los Alamos Scientific Laboratory, studying the light induced by electron bombardment of the atmosphere.

Hartman's work was complex, but he was never aloof. "Every year when the annual reviews came out and the salaries were negotiated, he always said we didn't have to worry; they were paying him more than he was worth already," said Barbara Freeman, his daughter. "He did the quiet supportive work that other individuals might not have been inclined to do."

That work included serving as faculty secretary and unofficial department diplomat. "This absence of any visible ego or sense of his own importance was a natural tool for bringing people together," said Cornell physics professor emeritus Don Holcomb.

Hartman was most at home in the laboratory. "He was a hands-on experimentalist," said Holcomb. "He did experiments in the old style, where you made things yourself." And he passed that ethic along to his students by leading the advanced physics laboratory, where upper-level undergraduates and graduate students conduct a range of classic and modern experiments.

"He ran the advanced laboratory in a beautiful way, always building new experiments to keep up with advancing physics research," said Corson. "I taught in that laboratory sometimes, under his leadership. It's a unique laboratory, and he had a great deal to do with building it."

Hartman also worked closely with founder Lloyd P. Smith to shape the educational philosophy of the School of Applied and Engineering Physics, and he served as associate director from 1971 until his retirement in 1983. That year, the Hartman Prize for outstanding experimental skills by a graduating physics student was established in his honor.

After he retired, Hartman penned "The Cornell Physics Department: Recollections and a History of Sorts" and "A Memoir on The Physical Review: A history of the first hundred years." Both are lively with anecdotes, written in the first person in the colloquial style that Holcomb called "pure Hartman."

Hartman wrote his own obituary for The Ithaca Journal. He referred to himself simply as Paul. He mentioned his love for astronomy and camping, his winemaking experiments and his volunteer work for the Red Cross.

"He was a very kind man," said Corson. "I never heard him say an ill word about anybody. Paul was 100 percent human being."

Hartman is survived by his wife, Margaret; a brother, David of Jamesville, N.Y.; three daughters: Barbara H. Freeman of Cape Elizabeth, Maine, Laurel L. Hartman of Ithaca and Sara W. Hartman of Maynard, Mass.; two grandchildren and their spouses; and four great-granddaughters.

A memorial gathering is planned at Kendal at Ithaca on Saturday, July 9, at 3 p.m.