Hubble and Slipher's Early Work on the Redshift Law

In 1929, Hubble used data from Esto Slipher without direct citation or acknowledgement. In 1953 he wrote to Slipher asking for some slides of his first 1912 spectrum of the radial velocity of the Andromeda Nebula. By regard, such first steps as by far the most important of all, wrote Hubble,. once the field is open to others can follow.