The collection includes research files, notes on research projects, general correspondence files from 1945-1975, and personal correspondence files from 1922 onwards (e.g. with Bok, Van de Kamp, Kuiper, Van Rhijn, Baade, Struve, Blaauw); correspondence and papers on the organization of astronomy and on the establishment of the Leiden Southern Station at Hartebeespoortdam (South Africa), the radio observatories at Dwingeloo and Westerbork, the European Southern Observatory; the establishment of "Astronomy and Astrophysics, a European Journal." Agendas, minutes and correspondence concerning the Faculty of Mathematical and Physical Sciences and the Senate of Leiden University, the Leiden Academic Arts Centre, the Leiden Observatory Fund, the Committee for Geodesy and Space Research (GROC), the Board of Curators of the Royal Netherlands Meteorological Institute, the Academic Council, and the Royal Dutch Academy of Sciences. Also lecture notes (both University and Popular Lectures), preparatory notes for symposia etc., records and diaries of foreign visits, and notes on discussions with colleagues, etc.