London Royal Society, November 11. F. W. ASTON: A second-order focusing mass spectrograph and isotopic weights by the doublet method. Several improvements have been embodied in this instrument, including adjustable slits and second-order focusing. The instrument has a resolving power of 2,000 and an accuracy of measurement approaching 1 in 106. By its means, a large number of natural doublets have been measured. In many of these, one member is a hydrocarbon molecule, hence the isotopic weight of 12C has been ascertained with particular care. From the results obtained the packing fractions and isotopic weights of more than twenty atomic species are tabulated, some for the first time.