[5] This estimate by Seares is based on a revision of Fath’s counts of nebulae in Selected Areas (Mt. Wilson Contr., No. 297; Astrophysical Journal, 62, 168, 1925).
[6] “A General Study of Diffuse Galactic Nebulae,” Mt. Wilson Contr., No. 241; Astrophysical Journal, 56, 162, 1922.
[7] The classification was presented in the form of a memorandum to the Commission on Nebulae of the International Astronomical Union in 1923. Copies of the memorandum were distributed by the chairman to all members of the Commission. The classification was discussed at the Cambridge meeting in 1925, and has been published in an account of the meeting by Mrs. Roberts in L’Astronomie, 40, 169, 1926. Further consideration of the matter was left to a subcommittee, with a resolution that the adopted system should be as purely descriptive as possible, and free from any terms suggesting order of physical development (Transactions of the I.A.U., 2, 1925). Mrs. Roberts’ report also indicates the preference of the Commission for the term “extra-galactic” in place of the original, and then necessarily non-committal, “non-galactic.”
Meanwhile K. Lundmark, who was present at the Cambridge meeting and has since been appointed a member of the Commission, has recently published (Arkiv für Matematik, Astronomi och Fysik, Band 19B, No. 8, 1926) a classification, which, except for nomenclature, is practically identical with that submitted by me. Dr. Lundmark makes no acknowledgments or references to the discussions of the Commission other than those for the use of the term “galactic.”
[8] Problems of Cosmogony and Stellar Dynamics, 1919.
[9] N.G.C. 4486 (M 87) may be an exception. On the best photographs made with the 100-inch reflector, numerous exceedingly faint images, apparently of stars, are found around the periphery. It was among these that Belanowsky’s nova of 1919 appeared. The observations are described in Publications of the Astronomical Society of the Pacific, 35, 261, 1923.
[10] “Early” and “late,” in spite of their temporal connotations, appear to be the most convenient adjectives available for describing relative positions in the sequence. This sequence of structural forms is an observed phenomenon. As will be shown later in the discussion, it exhibits a smooth progression in nuclear luminosity, surface brightness, degree of flattening, major diameters, resolution, and complexity. An antithetical pair of adjectives denoting relative positions in the sequence is desirable for many reasons, but none of the progressive characteristics are well adapted for the purpose. Terms which apply to series in general are available, however, and of these “early” and “late” are the most suitable. They can be assumed to express a progression from simple to complex forms.
An accepted precedent for this usage is found in the series of stellar spectral types. There also the progression is assumed to be from the simple to the complex, and in view of the great convenience of the terms “early” and “late,” the temporal connotations, after a full consideration of their possible consequences, have been deliberately disregarded.
[11] Publications of the Lick Observatory, 13, 12, 1918.
[12] Hβ is brighter than N2. Patches with similar spectra are often found in the arms of late-type spirals—N.G.C. 253, M 33, M 101. The typical planetary spectrum, where Hβ is fainter than N2, is found in the rare cases of apparently stellar nuclei of spirals; for instance, in N.G.C. 1068, 4051, and 4151. Here also the emission spectra are localized and do not extend over the nebulae.