Every day thousands of people worldwide consult Roget\'s Thesaurus—yet Peter Mark Roget, one of the most remarkable men of the 19th century, did not even begin the great work of classification which bears his name until he was 70. Before that, Roget had already made his own contributions to knowledge in a dozen different fields such as anatomy, mathematics, education, and optics, including papers on persistence of vision which had later impact on the development of motion pictures. From his involvement in the foundation of the University of London to his books on magnetism, galvanism, and physiology, this biography reveals the full story of Roget\'s impact on the great issues and the great personalities of the 19th century, and recounts the forgotten life behind one of the most famous of all reference books.