Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin, pp.2.56-2.57 [1]. His article entitled The radial velocity of the Andromeda Nebula reports making the first Doppler measurement on September 17, 1912. In his report, Slipher writes: "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." Three years later, in the journal Popular Astronomy, Vol. 23, p. 21–24 [2], Slipher wrote a review entitled Spectrographic Observations of Nebulae. In it he states, "The early discovery that the great Andromeda spiral had the quite exceptional velocity of - 300 km(/s) showed the means then available, capable of investigating not only the spectra of the spirals but their velocities as well." Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable "positive" (that is recessional) velocities.
W. de Sitter, "On distance, magnitude, and related quantities in an expanding universe, (1934) Bulletin of the Astronomical Institutes of the Netherlands, Vol. 7, p.205. He writes: "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"
