6 



IJISCOVERY 



"A knowledge of the construction of the heavens," he 

 wrote in 1811, " has always been the ultimate object 

 of my investigations." In 1784, in his first paper on 

 the subject, he described his method of star-gauging, 

 which, he said, " consists in repeatedly taking the 

 number of stars in ten fields of view of my reflector 

 very near each other, and by adding their sums and 

 cutting off one decimal on the right, a mean of the 

 contents of the heavens, in all the parts which are 

 thus gauged, is obtained." In 1785 Herschel outlined 

 his theory of the universe. " That the Milky Way," 

 he said, " is a most extensive stratum of stars of 

 various sizes admits no Innpcr of the least doubt, and 



SIR W. IIIvRSlHUI.. 

 By kind pcrmii^ioii ul Ihc S.l'.C.K. 



that our sun is actually one of the heavenly bodies 

 belonging to it is evident." The distant nebute were 

 looked upon as external universes, and in 1785 he 

 remarked that he had discovered 1,500 universes. 

 Herschel made two important assumptions — that his 

 gauging telescope was sufficiently powerful to penetrate 

 to the limits of this stratum, and that the stars were 

 distributed with some approach to uniformity. Ob- 

 viously, if these assumptions are granted, the number 

 of stars in any given region of the heavens becomes a 

 criterion of the depth of the stratum in that direction. 

 Herschel was thus enabled to estimate the possible 

 dimensions and shape of the sidereal system. He 

 sketched it as a cloven disc of irregular outline, extend- 



ing much farther in the direction of the Milky Way or 

 Galaxy than in that of the galactic pwles, the cleft re- 

 presenting the division in the galactic stream. The 

 sun he believed to be almost, but not quite, centrally 

 placed, being slightly nearer to the north galactic 

 pole than to the south, and perceptibly closer to the 

 boundaries of the system in the direction of Cania 

 Major than of Aquila. 



On the disc-theory, then, the sidereal system was 

 believed to be of limited, though vast, dimensions, and 

 the incbuh-e w^ere regarded as external universes which 

 appeared irresolvable through immensity of distance. 

 As his investigations progressed, Herschel was obliged 

 to modify his earlj' conclusions. He came to beUeve, 

 correctly, that many of the nebula; were gaseous 

 objects, and not clusters of stars at vast distances. In 

 1811 he definitely abandoned his belief — never very 

 confidently held — of an equal scattering of stars. " An 

 equal scattering of stars," he said, " may be admitted 

 in certain calculations ; but when we examine the Milky 

 Way or the closel}' compressed clusters of stars, of 

 which my catalogues have recorded so many instances, 

 this supposed equality of scattering must be given up." 

 He did not altogether abandon the supposition, for 

 " a certain properly modified equality of scattering " 

 formed the basis of the second method of star-gauging, 

 which he applied to the Galaxy in 1818. The first 

 disc-theory, however, was abandoned, and Herschel 

 did not put forward another definite hypothesis. At 

 the close of his career he regarded the sidereal system 

 as of greater extension in the direction of the Galaxy 

 than he had formerly believed. He found that " the 

 utmost stretch of the space-penetrating power of the 

 20-foot telescope could not fathom the profundity of 

 the Milky Way," and he maintained that," when our 

 gauges will no longer resolve the Milkj' Way into stars, 

 it is not because its nature is ambiguous, but because 

 it is fathomless." At the close of his investigations, 

 therefore, Herschel viewed the universe as a more 

 iiattened disc than he did at the beginning, and he 

 appears still to have looked on the Milky Way as in the 

 main an optical phenomenon, while conceding the 

 objective reality of the regions of streaming and cluster- 

 ing. 



Herschel's successor in the study of star-gauging was 

 his son. Sir John Herschel, who completed his father's 

 work by a survey of the southern hemisphere. Sir 

 John Herschel, however, does not appear to have been 

 aware of the gradual progression in his father's views, 

 for in his Outlines of Astronomy he reproduced the disc- 

 theory of 1785, referring to it as " the view of the con- 

 struction of the starry firmament taken by Sir Wilham 

 Herschel." The younger Herschel nowhere put for- 

 ward a definite theory of the structure of the universe. 

 He did not, however, adhere slavishly to the views put 



