Não informado. (1921). Bulletin of the Astronomical Institutes of the Netherlands. Astronomical Institutes of the Netherlands.
Chicago-referens (17:e uppl.)Não informado. Bulletin of the Astronomical Institutes of the Netherlands. Amsterdam: Astronomical Institutes of the Netherlands, 1921.
MLA-referens (8:e uppl.)Não informado. Bulletin of the Astronomical Institutes of the Netherlands. Astronomical Institutes of the Netherlands, 1921.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.