Jacobus Kapteyn

Jacobus Cornelius Kapteyn was a Dutch astronomer. He carried out extensive studies of the Milky Way. He found that the apparent movement of stars was not randomly distributed but had two preferential directions: the two star streams. This discovery was later reinterpreted as evidence for galactic rotation. Kapteyn also suggested that these stellar velocities could be used to find the amount of non-luminous matter in the galaxy, which his student, Jan Oort, measured in 1932, referring to it as "invisible matter".