Abstract

DURING THE SECOND DECADE of the twentieth century at least three different hypotheses involving the radial velocities of spiral nebulae appeared in astronomical journals, marking the introduction of a new field of study for astronomers. Willem de Sitter argued that a velocity-distance relation would exist if space were curved.' W. W. Campbell speculated on a possible link between radial velocities and evolutionary stages of nebulae.2 And V. M. Slipher announced his belief that the radial velocities of spiral nebulae were a reflection of our galaxy's motion relative to the nebulae. 3 The hypotheses were similar in that each created a theoretical framework within which the measurement of radial velocities of spiral nebulae was viewed as important. The origins of the hypotheses and their consequences for developments n astronomy were, however, dissimilar.