11Publications of the Lick Observatory,13, 12, 1918.
12Hβis brighter thanN2. 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, whereHβis fainter thanN2, 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.
13Monthly Notices.74, 699, 1914.
14Annalen der Wiener Sternwarte,20, 1907.
15Astronomische Nachrichten,214, 425, 1921.
16Publications of the Lick Observatory,13, 1918.
17SinceCis constant for all nebulae in a given class, the linear relation betweenΔ logdandCfor the different classes is something more than a mere geometrical relation arising from the observed equality of the meanmTin the various classes.
18This is apparent even among the observed classes. Referring toformula (3),mT+ 5 logbwill be constant in so far asCe+ 5 log (1 –e)is constant. The following table indicates that the latter term is approximately constant throughout the sequence of elliptical nebulae. The values ofCewere read from the smooth curve inFig. 6.
19Mt. Wilson Contr., No. 191;Astrophysical Journal,52, 162, 1920.
20Pease,Mt. Wilson Comm., No. 51;Proceedings of the National Academy of Sciences,4, 21, 1918.
21Pease,Mt. Wilson Comm., No. 32:ibid.,2, 517, 1916.
22Astrophysical Journal,55, 406, 1922.
23Monthly Notices,82, 133, 1922.
24Astronomical Journal,28, 75, 1914.
25Mt. Wilson Contr., No. 297;Astrophysical Journal,62, 168, 1925.
26The latest and most reliable results bearing on the distribution of faint (hence apparently distant) nebulae are found in Seares’s revision and discussion of the counts made by Fath on plates of the Selected Areas with the 60-inch reflector. When the influence of the cluster in Virgo is eliminated the density appears to be roughly uniform for all latitudes greater than about 25°.
27Monthly Notices,78, 3, 1917.
28Haas,Introduction to Theoretical Physics,2, 373, 1925.
Transcriber’s NotesText notes:For the HTML version, page numbers of the original printed text are displayed within braces to the side of the text. The numbers have been remapped to start at page 1.Footnotes have been renumbered and moved to the end of the paper.Tables have been moved to avoid splitting paragraphs. In some cases the tables have been reconfigured or split in order to work better with e-readers. As a result, page numbers may not be accurate for tables and adjacent text.Some of the mathematical equations were created using TeX notation that was converted into SVG images via the tex2svg function in MathJax. The Tex notation is contained in the plain text version.Except as mentioned above every effort has been made to replicate this text as faithfully as possible, including non-standard punctuation, inconsistently hyphenated words, etc.The original printed text is available from HathiTrust at:https://hdl.handle.net/2027/uc1.b3805680 (see pg 353).The outline “CLASSIFICATION OF NEBULAE” starting onpage 3lists N.G.C. 2117 as a type E7 nebula. The version of this paper printed inContributions from the Mount Wilson ObservatoryNo. 324 lists N.G.C 3115. Otherwise the papers are essentially identical.
Transcriber’s Notes
Text notes: