Slipher and the Nature of the Nebulae
  Why do some discoveries, which appear in hindsight to be obviously major
discoveries, have so little impact when they were made. These are discoveries
that were ahead of their time, but for some reason the scientific community was
not ready to absorb them. I got interested in Slipher after learning that by
1914 he had observed nebular redshifts up to 1000 km/s. Why did this not
convince people at the time that the nebulae were extragalactic ? Was this
another example, like Zwicky's (1933) discovery of dark matter in the Coma
cluster, of a discovery which was too far ahead of its time ? I conclude that
Slipher's situation was different from Zwicky's. The significance of Slipher's
nebular redshifts was partly recognised at the time, but I believe that its
full significance was masked by van Maanen's work which turned out to be
erroneous and reduced the impact of Slipher's discoveries.
