On _variable stars_ there is published every year by HARTWIG in the “Vierteljahrschrift der astronomischen Gesellschaft” a catalogue of all known variable stars with needful information about their minima &c. This is the completest and most reliable of such catalogues, and is always up to date. A complete historical catalogue of the variables is given in “Geschichte und Literatur des Lichtwechsels der bis Ende 1915 als sicher veränderlich anerkannten Sterne nebst einem Katalog der Elemente ihres Lichtwechsels” von G. MÜLLER und E. HARTWIG. Leipzig 1918, 1920. On _nebulae_ we have the excellent catalogues of DREYER, the “New General Catalogue” (N. G. C.) of 1890 in the “Memoirs of the Astronomical Society” vol. 49, the “Index catalogues” (I. C.) in the same memoirs, vols. 51 and 59 (1895 and 1908). These catalogues contain all together 13226 objects. Regarding other special attributes I refer in the first place to the important Annals of the Harvard Observatory. Other references will be given in the following, as need arises. FOOTNOTES: [Footnote 7: “Bonner Sternverzeichnis” in den Astronomischen Beobachtungen auf der Sternwarte zu Bonn, Dritter bis Fünfter Band. Bonn 1859-62.] [Footnote 8: “Bonner Durchmusterung”, Vierte Sektion. Achter Band der Astronomischen Beobachtungen zu Bonn, 1886.] [Footnote 9: “The Cape Photographic Durchmusterung” by DAVID GILL and J. C. KAPTEYN, Annals of the Cape Observatory, vol. III-V (1896-1900).] [Footnote 10: “Cordoba Durchmusterung” by J. THOME. Results of the National Argentine Observatory, vol. 16, 17, 18, 21 (1894-1914).] [Footnote 11: Aph. J., vol. 36.] [Footnote 12: H. A., vol. 71.] [Footnote 13: H. A., vol. 91, 92, 93, 94.] [Footnote 14: A catalogue of radial velocities has this year been published by J. VOUTE, embracing 2071 stars. “First catalogue of radial velocities”, by J. VOUTE. Weltevreden, 1920.] CHAPTER III. SOME GROUPS OF KNOWN STARS. 29. The number of cases in which all the eight attributes of the stars discussed in the first chapter are well known for one star is very small, and certainly does not exceed one hundred. These cases refer principally to such stars as are characterized either by great brilliancy or by a great proper motion. The principal reason why these stars are better known than others is that they lie rather near our solar system. Before passing on to consider the stars from more general statistical points of view, it may therefore be of interest first to make ourselves familiar with these well-known stars, strongly emphasizing, however, the exceptional character of these stars, and carefully avoiding any generalization from the attributes we shall here find. 30. _The apparently brightest stars._ We begin with these objects so well known to every lover of the stellar sky. The following table contains all stars the apparent visual magnitude of which is brighter than 1m.5.