V.M. Slipher's discovery of the rotation of spiral nebulae and the controversy with Bertil Lindblad

A study of the Lowell Observatory Archives has revealed that, starting in 1917, V.M. Slipher worked out a coherent hypothesis concerning the rotation of spiral nebulae. When Hubble became interested in the question in 1932, he discovered Slipher's work, studied it and concluded, according to his own observations, that Slipher was right. Slipher's conclusions concerning the dynamics of the spirals were only called into question in 1940 when Lindblad claimed that the spiral arms were `leading'. Hubble defended Slipher's conclusion, and Slipher himself intervened in the debate in 1944 with only a short note. The debate ended in part with the evolution of Lindblad's work and with the development of the density wave theory in the 1970s.