Jacobus Kapteyn (Sources)

1. "Jacobus C. Kapteyn." Jacobus C. Kapteyn. Soylent Communications, 2014. Web. 23 Feb. 2014. <http://www.nndb.com/people/657/000170147/>

2. "Jacobus C. Kapteyn." The Bruce Medalists: Jacobus C. Kapteyn. The Bruce Medalists, 24 Sept. 2013. Web. 23 Feb. 2014. <http://www.phys-astro.sonoma.edu/BruceMedalists/Kapteyn/>