de Sitter, W. (1934). „On distance, magnitude, and related quantities in an expanding universe”. Bulletin of the Astronomical Institutes of the Netherlands. 7: 205. Bibcode:1934BAN.....7..205D. „It thus becomes urgent to investigate the effect of the redshift and of the metric of the universe on the apparent magnitude and observed numbers of nebulae of given magnitude”
Slipher, Vesto (1912). „The radial velocity of the Andromeda Nebula”. Lowell Observatory Bulletin. 1: 2.56—2.57. Bibcode:1913LowOB...2...56S. „The magnitude of this velocity, which is the greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at present no other interpretation for it”