NEBUL.E. In addition to the stellar systems. we have in the sky another interesting class of objects, the nebula (q.v.). These appear in the telescope as hazy clouds of light, usually con densing here and there into brighter nuclei. It is impossible to escape the conviction that these nebnIce are gaseous. and that they furnish a sort of key to the life history of the ordinary stars. indeed, spectroscopic evidence tends to show that many of the nebula are composed of matter in the form of incandescent gas. What, then, can be more plausible than to see in the stars the • result of a gradual process of cooling in nebulous matter? One can imagine readily that con densation and contraction might result in the formation of a single central sun, or that there might be two or more such centres. indeed, the sky actually does show some eases of 'double nebula.: 'Recent mathematical researches have indicated that the tidal effects seen on a small scale in the case of the earth and moon would play a much more important part in double nebuhp while still in the plastic condition. The mighty cosmic tides that would there be set up might undoubtedly produce perturbations great enough to account for the most complete changes in the character of motion within the system. An account of this study of the past and future history of cosmic systems may be found in the article COSMOGONY.
BIBLIOGRAPHY. The following hooks will be Bibliography. The following hooks will be found useful in a more detailed study of the science of astronomy: Grant, History of Physical Astronomy (London, ; chauvenet, Manual of Spherigal and Prae .tical Astronomy (Philadelphia, 1863) ; Young, The Sun (New York. 1881) ; Newcomb, Popular As tronomy (New York, 1882) ; Doolittle, Treatise on Practical Astronomy Applied to Geodesy and Navi gation (New York, 1885) ; Young, Text-book of General Astronomy (New York, 1888) Tisserand, Trait(' de in&anique caeste, 4 vols. (Paris, 1889 96) ; Kayser and Runge, Ueber die Spectre?? der Elements, 7 vols. (Berlin, 1888-93) ; Clerke, Sys tems of Stars (New York, 1890) ; Oppolzer, Lehrbuch zur Bahnbestimmung (Leipzig, 1882) ; Scheiner, Die Spcktralanalyse der Gestirne (Leipzig, 1890) : Ball, Atlas of Astronomy (New York, 1893) ; Valentiner, Handwarterbuch der Astronomic (Karlsruhe, 1896) ; Berry, Short History of Astronomy (London, 1898) ; Darwin, The Tides, end Kindred Phenomena in the Solar System (Boston, 1898) ; Newcomb, Elements of Astronomy (New York, 1900) : Newcomb, The Stars (New York, 1901) ; Clerke, Popular His tory of Astronomy in the Nineteenth Century (New York, 1893).