Dennison Bancroft, a distinguished member of Colby's Physics Department, died January 19, 1999, in Exeter, N.H., at 87. A 1933 Phi Beta Kappa graduate of Amherst, he earned his doctorate from Harvard in 1939. After two years as a research associate in geophysics at Harvard, he enlisted in the Navy and was eventually transferred to work at Los Alamos until the end of the war.     He taught physics at Princeton University and Swarthmore College before coming to Colby in 1959. While serving the College as department chair, he introduced a continuing series of laboratory experiments for physics majors. He received sizable grants from the National Science Foundation to investigate the velocity of sound in gases, and he also worked in plasma physics.     A master of several trades, Bancroft built his own cottage, was an accomplished skater and pistol marksman, and a skillful navigator. Even as an undergraduate he was a competitive sailor, and he retained a lifelong love of sailing; after retiring he captained his boat across the Atlantic to the Mediterranean Sea and around Europe. He is survived by his wife, Marjorie, twin sons, a daughter, nine grandchildren, four great-grandchildren, two stepsons, three stepdaughters, a sister and several nieces and nephews.