Thomas Foxen Anderson, 80, a scientist who used the electron microscope to take historic photos of mating bacteria and viruses infecting a host cell, died on August 11, 1991 at Jeanes Hospital.   He was a biophysical chemist and geneticist who was among the first to use the electron microscope in the study of viruses. He eared his doctorate in chemistry from California Institute of Technology.  He joined the faculty of the University of Pennsylvania in 1942 and retired from Penn in 1977.  He had a long association with the Fox Chase Cancer Center and joined the center’s staff in 1954 and served until 1983, when he became a senior member emeritus.  He was elected to the National Academy of Sciences in 1964.  He was a former president of the International Federation of Electron Microscope Societies and former president of the Electron Microscope Society of America.    He was chairman of the US National Committee of the International Union  for Pure and Applied Biophysics from 1965-1969.  From 1985 to 1988, he served as chairman of the genetics section of the U.S. National Academy of Sciences.