DISCOVERY 



forward by liis father. Indeed, in 1836 he wrote from 

 South Africa to Sir WilHam Rowan Hamilton : " It is 

 impossible to resist the conviction that the Milky Way 

 is not a stratum but a ring." 



The first astronomer who made an exhaustive study 

 of the elder Herschels papers and noticed his reitera- 

 tions of opinion was Wilhelm Struve, the German 

 director of the Russian National Observatory at 

 Pulkova. Struve was also the direct successor of 

 Herschel as an original investigator of the problem. 

 His own study of stellar distribution led to the formula- 

 tion in 1847 of his modification of the original disc- 

 theory. The universe was, he believed, of finite thick- 

 ness — the stars being obviously limited in number 

 in the direction of the galactic poles — but of infinite 

 extension in the direction of the Galaxy. He main- 

 tained, indeed, that this was borne out by Herschel's 

 later investigations, and that when Herschel spoke of 

 the Galaxy as fathomless, he meant that it was un- 

 fathomable. Along with this view of an infinite 

 extension in the galactic plane, Struve advanced his 

 theor}' of the extinction of light in space — maintaining 

 that the more distant of the galactic star-clouds were 

 rendered invisible by the extinction of light. Struve's 

 hypothesis was rejected by the majority of his con- 

 temporaries ; and their rejection has been justified by 

 subsequent research. Grant and the younger Herschel 

 showed that in important respects it failed to explain 

 the observed facts ; while Encke demonstrated that 

 the theory was based on five assumptions, all of which 

 were questionable. 



The work of Proctor in England may be regarded as 

 analogous to that of Struve on the Continent. He made 

 an exhaustive study of Herschel's papers, and proved 

 beyond a doubt that the detailed disc-theory of 

 1785, with its corollary — the hypothesis of island 

 universes — had been definitelj' abandoned by Herschel. 

 His own views of the universe represented a reaction 

 from the views of Struve ; he regarded the universe as 

 much less extended in the plane of the Galaxy than 

 Struve, or even Herschel, had believed. " Where," 

 he wrote, " Herschel thought he was penetrating to 

 the extreme limits of the sidereal system, he w^as in 

 reality only anah'sing more and more searchinglj' an 

 aggregation in which many orders of stars were mixed 

 up. What he failed to do was not, as he supposed, 

 to sound the Galaxy, but to recognise as separate stars 

 the minutest order of orbs included within such aggre- 

 gations." Proctor beUeved the stars composing the 

 Milky Way to be smaller and" more closely crowded 

 together than those in the vicinity of the sun. In his 

 view, therefore, the extent of the sidereal system was 

 much less than Herschel had believed it to be, although 

 he admitted that w^e " are still bound to accept a con- 

 siderable extension of our system in the galactic plane 



as fairly deducible from the gauges of Sir W. 

 Herschel." 



In 1870 Proctor plotted on a single chart all the stars, 

 to the number of 324,198, contained in Argelander's 

 Durchmusterung. This chart revealed the aggregations 

 of the brighter stars towards tha galactic plane — a 

 discovery abundantly verified by the independent work, 

 years afterwards, of Schiaparelli and Gore — and also 

 the connection between bright stars and the nebulous 

 light of the galactic background. " It is," he said. 

 " utterly impossible that excessively remote stars 

 could seem to be clustered exactly where relatively 

 near stars are richly spread." The general conclusion 

 which Proctor reached was that " the sidereal system 

 is altogether more complicated, altogether more varied 

 in structure, than has hitherto been supposed. Within 

 one and the same region coexist stars of many orders 

 of real magnitude, the greatest being thousands of 

 times larger than the least. All the nebul.c hitherto 

 discovered, whether gaseous or stellar, irregular, 

 planetary, ring-formed, or elliptic, exist within the limits 

 of the sidereal system. They all form part and parcel 

 of that wonderful system whose nearer and brighter 

 parts constitute the glories of our nocturnal heavens." 



The excess of bright stars on the Galaxy was inter- 

 preted by Proctor as indicating that the bright and faint 

 stars were comparatively- close together, and in this he 

 was followed by Gore. Celoria, however, interpreted 

 the fact as indicating the existence of two galactic 

 rings, w^hile Gould believed the " secondary' galaxy " 

 of lucid stars to be due to the existence of a flattened 

 cluster of bright stars comparatively close to the sun. 

 This suggestion, however, did not receive much 

 attention, and was rejected by both Gore and Newcomb. 

 The former astronomer, writing in 1898, regarding 

 Kapteyn's revival of Gould's suggestion in another 

 form, expressed the view that "the sun is a member of a 

 cluster of stars, possibly distributed in the form of a 

 ring, and that outside this ring, at a much greater 

 distance from us than the stars of the solar cluster, 

 lies a considerably richer ring-shaped cluster, the light 

 of which, reduced to nebulosity by immensity of 

 distance, produces the Milky Way gleam of our mid- 

 night skies." 



This conclusion, however, was not altogether sup- 

 ported by the exhaustive investigations on stellar 

 distribution carried out by Seeliger, of Munich, from 

 1884 to 1S98. Newcomb, writing in 1901, referred to 

 these researches as " the most thorough study of the 

 distribution of the great mass of stars relative to the 

 galactic plane." Dealing with over 100,000 stars, 

 Seeliger demonstrated the gradual increase of the 

 number of stars from each of the galactic poles to the 

 Milky Way. Now, if the Galaxy were simply a ring 

 of stars surrounding a star-sphere, the number of stars 



