This argument before the Board of Examiners-in-Chief of the U.S. Patent Office contains full or partial transcriptions of eight letters from Edison to former U.S. Senator Thomas L. Clingman written during the period March-December 1879. In April 1880 Clingman's application for an incandescent lamp filament composed of zirconia and other substances had been rejected by the Patent Office, partly on the grounds that it conflicted with Edison's U.S. Patent 219,628. Clingman presented his correspondence with Edison to support his argument that the inventor had no interest in zirconia as an incandescing substance. Although the Board rejected Clingman's application, after considerable legal wrangling it was accepted in 1883 and issued as U.S. Patent 276,133. Courtesy of the National Archives.