Harlow Shapley
In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate), concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.