The ground level of Th I 6d27s2 3F2 and four other low even levels were first determined by Schuurmans (46.3) from two dozens of high odd levels. A considerable extension of the system 6d27s2+6d37s by Zalubas (59.2) left the odd levels without an interpretation. By means of extended observations in the infrared, Steers (67.5) could find some low odd levels which were confirmed and labeled by Zalubas (68.1) assisted theoretically by Trees (60.6). The interpretation of Fourier transform recordings in the infrared region led Giacchetti and Blaise (70.10) to find new levels partly reported by Brewer (71.1). Of the 3100 infrared lines (3000 - 924 nm) reported in (74.8), 1900 lines are classified as Th I and improve the earlier measurements quoted in (67.5) and (71.7). Zalubas collected his new measurements with earlier data and mentionned a list of 35000 lines covering the range 200-2970 nm (76.8). About 9500 classified lines, Zeeman effect data for 2281 lines and 776 levels were reported in this publication.
The thorium spectrum emitted by a hollow cathode was reobserved by Fourier transform spectrometry at Kitt Peak National Observatory from 1350 to 292 nm (83.9) and extended isotope shift data DT (232-230) (83.2) were used to improve the configuration assignment (83.5). From the isotope shift of unclassified lines, the excited configuration 6d4 starting at 21176 cm-1 was found and included in the parametric interpretation of the mixed configurations (6d+7s)4. The wavefunctions partly reported below were used to study crossed second order effects of isotope shift (88.5). The level energies quoted below are recalculated (unp .2) from the wavenumbers of (83.9). The associated isotope shifts are taken from (83.2) and (88.5) with some additional values (unp.18).