Henry Gordon Gale (1874-1942, left), US astrophysicist Arthur Jeffrey Dempster (1886-1950, right), US-Canadian physicist, shaking hands. Dempster is best known for his work in mass spectrometry, the analysis of compounds by looking at the mass and charge ratios. He developed the first modern mass spectrometer, which was over 100 times more accurate than previous versions, and established the basic theory and design of mass spectrometers that continues to be applied today. Dempster's work on mass spectrometry led to his discovery of the radioactive uranium-235 isotope. Gale edited the 'Astrophysical Journal' from 1912 to 1940.