more remote than the rest, and belong, therefore, to the sidereal system. The fact that nebulae are rich in regions remote from the milky-way would confirm this supposition, if not of itself sufficient (as Herbert Spencer thinks) to establish it beyond question. If the nature of the Magel- lanic clouds had been known to Sir W. Herschel, this inference would have appeared to him irresistible. How ever, the results towards which his later views seemed tending were not definitely indicated or adopted by him, probably because he had already attained an extreme old nge when he first enunciated his later and juster views of the sidereal universe. Sir J. Herschel does not appear to have recognised his father s change of views, though con scious of serious difficulties in the older theory, and even definitely indicating the fact that the constitution of the Nubeculse cannot possibly be reconciled with that theory. The elder Struve, probably the only astronomer of his time who had thoroughly studied Sir W. Herschel s remarkable papers, recognised clearly the change in Herschel s opinions. Following a suggestion thrown out by Piazzi, Struve com pared the number of stars down to the eighth magnitude in different directions round the equator, and justly regarded the greater richness of such stars on and near the galaxy as a disproof of Herschel s earlier theory of generally uni form distribution. Strangely enough, however, while thus recognising a variation in the richness of stellar distribution in one direction, i.e., in approaching the medial plane of the galaxy, Struve was unable to divest himself of a belief in uniformity of distribution in directions parallel to that plane. In an investigation claiming to be free from all hypothesis, but in reality (as Encke, Forbes, Proctor, and others have shown) based on several hypotheses, some of which are not even probable, Struve advanced the theory that the sidereal system is infinite in extension along the
direction of the medial plane of the galaxy.But in reality the evidence we possess indicates laws of stellar aggregation which by their very nature preclude the possibility of applying such methods of gauging as either Sir W. Herschel or W. Struve endeavoured to use. The gathering of stars of the leading orders of apparent magni tude in the galactic zone shows that stars of many orders of real size and brightness are there gathered together. The analysis of a rich star-region with higher and higher powers is shown to be, not necessarily, as was supposed, the pene trating farther and farther into space, but the more and more searching scrutiny of one and the same region of space. The two processes, indeed, may be combined, an increase of telescopic power bringing into view at the same time smaller stars in a particular region and remoter stars lying towards the same direction. In fact, it would be as great a mistake to assume, without definite evidence, that new stars so revealed are smaller in real magnitude, as to assume that they are more remote. The only kind of evidence available to discriminate between the two explana tions, or to show to what extent either may operate, is that derived from statistical enumeration : but so complex are the relations involved, that such enumeration can only be interpreted when graphically illustrated. In other words, the secrets of the stellar universe can only be revealed by presenting in well-devised maps the results of widely ex tended scrutiny of the star depths. This process has already been applied by Mr Proctor to stars down to the eleventh order of magnitude (in the northern heavens), the resulting view of the stellar universe differing widely from that which would have been presented if any of the theories hereto fore advanced had been just. It is probable that an exten sion of the system of uniform star-gauging and charting to the remoter star-depths will still further illustrate the com plexity and diversity of structure existing within the universe. Already these general conclusions may be re garded as established: "The sidereal system is altogether more complicated and more varied in structure than has hitherto been supposed : in the same region of the stellar depths co-exist stars of many orders of real magnitude; all the nebulae, gaseous or stellar, planetary, ring-formed, elliptical, and spiral, exist within the limits of the sidereal system ; and lastly, the whole system is alive with move ments, the laws of which may one day be recognised, though at present they are too complex to be understood."
Bibliography.
Among the most important treatises relating to the history of astronomy may be mentioned Delambre s Histoire d Astronomie (Paris, 1817-21), Narrien s Origin and Progress of Astronomy, Lalaude s Bibliographic Astronomique, Grant s History of Physical Astronomy (London, 1852), Bailly s Histoires de I Astronomic Ar- cienne (Paris, 1781) et Mcdcrne (Paris, 1785), Loomis s History of Astronomy (New York, 1855), and the History of Astronomy in the Library of Useful Knowledge. But the reader desirous of becoming well acquainted with the modern developments of the science will find it necessary to study the papers published by societies specially devoted to astronomy, as well as the astronomical papers published by other scientific bodies. Amongst the proceedings, journals, &c., to be particularly examined are the following: -In Britain the Memoirs and Monthly Notices of the Astronomical Society, the Transactions and Proceedings of the Royal Societies of London, Edinburgh, and Dublin, and the Reports of the British Association; in France the Comptes Rcndus of the Paris Academy of Sciences ; in Germany the Abhandlungen and Monatslcricht of the Berlin Academy of Sciences, the Sitzungsberichte of the Imperial Academy of Vienna, the Vierteljahrsschrift of the Astronomischen Gesellschajt (Leipsic), and the Astronomische Nachrichtcn (Kiel) ; in Russia the Bulletins of the Imperial Academy of Sciences ; the proceedings published by learned bodies in Italy, Belgium, Portugal, and Den mark ; in America the Reports and Proceedings of the Smith sonian Institution (Washington), the Journal of the Franklin In stitute, Silliman s Journal, and the Proceedings of the American Association, &c. ; besides transactions published by various societies in Australia, Canada, and other colonies. The following are among the principal treatises on theoretical and physical astronomy : The Principia of Newton, Laplace s Exposition du Systeme du Monde and Traiti de Mecanique Celeste, Delambre s Astronomic Theo- retique, Brunnow s Spherical Astronomy, and the fine American works by Watson (of Ann Arbor), Chauvenet, and Loomis. The best works on practical astronomy are Pearson s Introduction to Practical Astronomy, the second volume of Chauvenet s work just mentioned (entitled Spherical and Practical Astronomy), and Loomis s Practical Astronomy. But, with the improvements con tinually being made in instruments and methods, the best work on practical astronomy soon becomes obsolete unless supplemented by the careful examination of the journals and proceedings mentioned above. For this reason a section on Practical Astronomy has not been added here, as in former editions of this work. Among popular treatises on general astronomy the most trustworthy are Airy s Ipswich Lectures, Arago s Astronomic Populairc (and the English version by Grant and Smyth), Herschel s Outlines of Astronomy -, Chambers s Descriptive Astronomy, and Guillemin s The Heavens (later editions). Among works in special departments of astronomy may be mentioned Nasmyth s treatise On the Moon, Secchi s Le Soldi, Herschel s Southern Observations, Pingre s Comctographie, Coulvier-Gravier s Rccherches sur les Etoiles Filantes, and W. Struve s Etudes d? Astronomic Stellaire. The principal star atlases are Harding s Atlas Novus Ccelestis, Heis s Atlas and accompanying Catalogue, Bode s Uranographia, Argelander s Uranometria, and Lubbock s six maps on the Gnomonic Projection, published for the S.D. U.K. For many departments of stellar research, however, celestial atlases are insufficient, and star catalogues have to be em ployed. Among the most important catalogues of isolated stars are Flamsteed s, Bradley s, Piazzi s, Lalande s, Groombridge s Circum- polar Stars, the Catalogue of the British Association, Argelander s Bonn series (containing 320,000 stars), Bessel s, Weisse s, Main s, &c. Among catalogues of double stars may be mentioned Sir W. Herschel s in the Phil. Transactions, those by Sir J. Herschel and South, and by Sir J. Herschel alone, the lists in the Monthly Notices of the Astr. Society, and the catalogues of Dembouski, Burnham, P. Secchi, Brothers, and others. Schellen s Spectral Analyse, or Roscoe s work on Spcctroscopic Analysis, will be suffi cient introduction to the subject of astronomical spectroscopy. In all the departments, however, there are many works besides those named which the student who desires to deal thoroughly with astronomy will do well to examine.
For the construction and use of astronomical instruments see the articles Observatory, Spectroscope, Telescope, &c.
(R. A. P.)
