Februakt 1, 1895.] 



KNOWLEDGE. 



35 



brought up to date. The book has grown in size, and by 

 " a process of assimilation rather than of mere accretion," 

 so well has the new matter been incorporated with the old. 

 Well arranged, and written in the authoress's agreeable 

 and lucid style, its wealth of accurate detail neither over- 

 powers nor confuses, and the volume is as fascinating as it 

 is instructive. 



The period with which the work deals has been a 

 memorable one in the history of astronomy. Planets and 

 satellites have been added to the solar system, and, through 

 the great improvement in all instruments, astronomers 

 have been able to carry their investigations to a degree of 

 completeness and accuracy formerly undreamed of. The 

 invention of the spectroscope has opened an entirely new 

 field of research, and its far-reaching discoveries are 

 probably only in their infancy. Photography has come 

 to our aid, giving us new views as to the character of the 

 nebulae, and has disclosed vast regions of space, peopled 

 with countless stars. The science, too, has greatly changed 

 its aspect. Once the study of the few, it has become of 

 the deepest interest to the many ; once considered from its 

 very completeness and perfection to be losing in interest, 

 it has disclosed infinite possibilities for the future. 



To give any detailed account of a work like the present 

 one is manifestly impossible ; we can only briefly touch 

 upon one or two points, and for the rest must refer our 

 readers to the book itself. 



The work appropriately opens with a resume of the 

 progress of our knowledge of the sidereal universe, and an 

 account of the discoveries of the elder Herschel, commenced 

 in 1774, and only ending with his life. When he began 

 " to explore with line and plummet the shining zone of 

 the Milky Way," he commenced with the convenient 

 assumptions that stars are equally distributed, and that 

 apparent brightness is an approximate measure of a star's 

 distance. The first assumption led to his adoption of the 

 theory of Thomas Wright, that the Milky Way " is the 

 projection on the sphere of a stratum or disc of stars (our 

 sun occupying a position near the centre), similar in 

 magnitude and distribution to the lucid orbs of the con- 

 stellations." Herschel was further led to assume that, 

 since his large telescope resolved numerous nebulse into 

 stars, all nebulfe would be resolvable with sufiicient optical 

 means. 



He soon saw that the assumption of an equal distribution 

 of stars must be given up : the detailed examination of 

 the Milky Way and its numerous associated clusters was 

 sufficient to disprove it ; and he described our sun and its 

 companion stars as surrounded by a magnificent collection 

 of innumerable stars, called the Milky Way — thus aban- 

 doning the " disc theory " of the universe, which, however, 

 long held its ground in the literature of the science. He 

 was also led to admit as probable that nil nebulfe were not 

 of a stellar nature ; but this question was only finally 

 answered by the spectroscope. 



Bessel adopted tlif amount of proper motion as a better 

 criterion of the distance of a star than its brightness. 

 It was impossible to resist the conclusion "that the 

 apparently swiftest moving stars are on the whole the 

 nearest to us," and the discovery of the swift move- 

 ment of the inconspicuous star 61 Cygni showed that the 

 fainter stars may be much nearer than others of greater 

 brightness. The parallax of stars, again, showed no relation 

 between nearness and brightness, Cancpus, Arcturus, and 

 Rigel being indefinitely remote, and exceeding our sun 

 perhaps two thousand times in brilliancy, "while many 

 inconspicuous objects, which prove to be in our relative 

 vicinity, must be notably his inferiors." It follows, therefore, 

 that stars are unequally distributed, and of unequal 



brightness, and if we consider the spectroscopic evidence 

 we reach still further conclusions : — 



" Brilliant suns are swayed from their courses by the 

 attractive power of massive yet faint luminous companions, 

 and suffer eclipse from obscure interpositions. Besides, 

 efl'ective lustre is now known to depend no less upon the 

 qualities of the investing atmosphere than upon the extent 

 and radiative power of the stellar surface. Red stars must 

 be far larger in proportion to the light difiused by them 

 than white or yellow stars. There can be no doubt that 

 our sun would at least double its brightness were the 

 absorption sufl'ered by its rays to be reduced to the Sirian 

 standard ; and, on the other hand, that it would lose half 

 its present efficiency as a light source, if the atmosphere 

 partially veiling its splendours were rendered as dense as 

 that of Aldebaran." 



The Milky Way can no longer be regarded as "the mere 

 visual effect of an enormously extended stratum of stars, 

 but as an actual aggregation, highly irregular in structure, 

 made up of stellar clouds and groups and nodosities." But 

 we cannot pursue this subject further, except to mention 

 the singular " dark holes and dusky lanes" shown on the 

 photographs by Barnard, Russell, and others, with which 

 the Milky Way is tunnelled and furrowed. These dark 

 spaces, which often appear to be bounded sharply by lines 

 of stars, present a difficult subject for investigation. It 

 seems practically certain that they cannot be casual inter- 

 spaces between irregularly scattered star masses. Such 

 spaces, with sharply defined edges, do not occur among 

 points scattered at random on a plane, and it is obviously 

 far more improbable that they should exist among stars 

 scattered through space, forming tunnels directed precisely 

 in the line of sight. 



Long ago, as has been stated, Sir W. Herschel was led 

 to conclude that all nebulae could not be wholly stellar, 

 and spectroscopic researches, commenced by Huggins in 

 1863, have abundantly proved the gaseous nature of many 

 of these objects. Not less important is the evidence of the 

 intimate connection between nebulas and stars. Dr. and 

 Mrs. Huggins, in 1888, found in the nebula of Orion 

 "four groups of fine bright lines, originating in the 

 continuous light of two of the trapezium stars, but extending 

 some way into the surrounding nebula." And from 

 Dr. Roberts's beautiful photographs of spiral nebula, with 

 stars arranged along the nebulous spirals, we are driven 

 to the same conclusion of intimate association. Indeed, 

 as Miss Gierke observes, in speaking of Dr. Common's 

 magnificent photograph of the nebula in Orion, " photo- 

 graphy may be said to have definitely assumed the office of 

 historiographer to the nebulae," and this method will 

 certainly, in time, set at rest all questions of variability in 

 form or brightness. Thus, putting aside such vexed 

 questions as that of the mysterious " chief nebular line," 

 some certain results have been arrived at with regard to 

 the true status of nebula. They can no longer be regarded 

 as remote worlds of stars, and if we " add the evidence 

 of the spectroscope to the effect that a large proportion of 

 these perplexing objects are gaseous, with the intimate 

 relation obviously subsisting between the mode of their 

 scattering and the lie of the Milky Way, it becomes 

 impossible to resist the conclusion that both nebular and 

 stellar systems are parts of a single scheme." 



The interesting chapter on solar theories is well fitted 

 to serve as a lesson in modesty, so diverse and conflicting 

 are the various hypotheses, so difficult to harmonize are 

 the observed facts. But amidst the doubt and confusion, 

 some conclusions seem tolerably certain. Tnus it seems 

 probable " First, that the sun is mainly a gaseous body ; 

 secondly, that its stores of heat are rendered available at 



