Slipher's 1917 paper was in many ways the culmination of his programme
of nebular spectroscopy. Despite almost all his redshifts being
positive, he did not announce the expansion of the universe, and it is
interesting to ask why this was. The main reason is that the depth of
his dataset was too shallow (although it was the same sample used by
Hubble in 1929), and the velocity field is dominated on these scales
by peculiar velocities. In this respect, Slipher's analysis and
conclusions seem strikingly modern. In particular, he measures the
motion of the Milky Way, and uses this in a marvelously subtle
argument to show that nebulae are probably distant stellar systems.