H. Zuidervaart, “P.C. van der Kruit, K. van Berkel, The legacy of J. C. Kapteyn. Studies on Kapteyn and the development of modern astronomy”, BMGN-LCHR, vol. 116, no. 4, pp. 543–544, Jan. 2001, doi: 10.18352/bmgn-lchr.5589.