Humason, M. L. (1932). "The Emission Spectrum of the Extra-Galactic Nebula N. G. C. 1275". Publications of the Astronomical Society of the Pacific. 44: 267. Bibcode:1932PASP…44..267H. doi:10.1086/124242.
Mayall, N. U. (1934). "The Spectrum of the Spiral Nebula NGC 4151". Publications of the Astronomical Society of the Pacific. 46: 134. Bibcode:1934PASP…46..134M. doi:10.1086/124429.
Humason, M. L. (1932). "The Emission Spectrum of the Extra-Galactic Nebula N. G. C. 1275". Publications of the Astronomical Society of the Pacific. 44: 267. Bibcode:1932PASP…44..267H. doi:10.1086/124242.
Mayall, N. U. (1934). "The Spectrum of the Spiral Nebula NGC 4151". Publications of the Astronomical Society of the Pacific. 46: 134. Bibcode:1934PASP…46..134M. doi:10.1086/124429.