Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/896

822 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: &quot;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.&quot;

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