Interferometric Wavelengths of Thorium Lines between 9050 and 2566 Å

486 vacuum wavelengths of the thorium spectrum in the range 9050–2566 Å have been measured by comparison with the isotope Kr 86 line at 6057 Å, by using a reflecting echelon interferometer crossed externally with a Littrow spectrograph. Tests of accuracy by recurring interlevel differences indicate that the average error is about 1 in 107. Details are given of the sealed-off Schüler tube used as the thorium source. Difficulties which arose from the use of an Ebert spectrograph to photograph the spectrum are discussed.