CHAP. III., 2.] 



ASTBONOMY. SIE WILLIAM HEBSCHEL. 



43 



tions on the other planets and on the moon we here 

 pass over for want of space. 



I have reserved the observations on the nature of 

 the sun to this place, because everything leads us to 

 assimilate the nature of the sun and of the fixed stars. 

 The belief that the luminous disk of the sun is a photo- 

 sphere or luminous atmosphere of great tenuity sur- 

 rounding a globe of comparative density and dark- 

 ness, was long anterior to Herschel, and in fact due 

 to Dr Patrick Wilson of Glasgow, whose admirable 

 paper on this subject was published in the Philoso- 

 phical Transactions for 1774, in which he explains 

 the phenomena of the solar spots by apertures in the 

 luminous atmosphere, discovering the dark nucleus 

 below, and some shell or shells of intermediate 

 brightness which form the penumbra. 1 These con- 

 clusions were most clearly and ably deduced from a 

 careful observation of the changing aspect of the spots, 

 as they move by the solar rotation from the centre 

 to the edge of his disk. It is to be regretted that 

 Herschel does not more pointedly refer to the dis- 

 coveries of Wilson, which were more than twenty 

 years antecedent to his first paper on the subject, 

 and with which he could hardly fail to have been ac- 

 quainted. A similar remark applies, in a less degree, 

 to his papers generally, which rarely contain references 

 to the observations and speculations of his predeces- 

 sors. Herschel adopted Wilson's hypothesis almost 

 literally, and his long series of patient observations on 

 the sun, made with high powers and at an eminent risk 

 to his eyesight, enabled him to classify the singularly 

 varied appearances of that wonderful orb, and to 

 draw some probable conclusions from the excessive 

 rapidity and seeming tumult of the exterior portions 

 of it. That the photosphere is strictly gaseous he 

 rendered very probable, an inference confirmed by 

 the direct observations of Arago as to the un- 

 polarized character of its light. The singular dis- 

 closure of faint red prominences extending far beyond 

 the disk, and observed in the total eclipses of 1842 

 and 1851, shows that there is still much which re- 

 gards the mysterious nature of the sun within reach 

 of direct observation ; and the same may be observed 

 of the direct experiments lately made on the heat 

 and light of different parts of the disk, which diminish 

 to one-half between the centre and the edge, and 

 appear to attain a maximum at the solar equator. 



A convenient, though not a strictly chronological 

 arrangement of Sir W. Herschel's more important 

 sidereal discoveries and speculations maybe made 

 under the following heads : 



I. Of double Stars and their mutual connection. 



II. Of the Nature of Nebulae, and the so-called 

 Nebular Hypothesis. 



III. Of the Grouping of the Stars generally in 

 space, and the significance of the Milky Way. 



IV. Of the Motion of our System in space. 



I. On Double Stars. Discovery of Binary Sys- 

 tems. Double stars were noticed as objects of curio- 

 sity even before the discovery of the telescope. The 

 group of the Pleiades attracted attention from the 

 earliest times. Amongst the earliest double stars 

 carefully observed were Ursae Majoris (by Kirch, 

 1700) ; a Centauri in 1709 ; y Virginis and Castor 

 by Bradley (1718 and 1719); Mayer made a con- 

 siderable catalogue of double stars in 1756. But 

 Lambert first announced in 1761 (in his Lettres Cos- 

 mologiques) the probability of the mutual revolution 

 of suns, in these remarkable words (speaking of clus- 

 ters of stars), " It will perhaps be decided whether 

 there are not fixed stars which make their revolutions 

 in no long periods round their common centres of 

 gravity." Mitchell, in 1767 and 1784, maintained 

 the same views, but supported them by an applica- 

 tion of the then young science of probability, hazard- 

 ous in its principle, and unquestionably wrong in its 

 numerical solution. 2 



Sir W. Herschel commenced his observations on 

 double stars with the hope of ascertaining the Annual 

 Parallax in the manner previously indicated by 

 Galileo and James Gregory ; but, as in many parallel 

 instances, whilst he failed of his main result, he dis- 

 covered unsought a phenomenon more unexpected 

 and probably more interesting. With the micro- 

 metrical means at his disposal, he entirely failed in 

 detecting any semi-annual fluctuation of the inter- 

 val between the members of the pair of stars, but 

 he found (in some instances) progressive and con- 

 tinually increasing changes both in the relative 

 position and distance of the two. He com- 

 menced his observations in 1779, but it was not 

 until 1802 that he thought himself entitled to an- 

 nounce with confidence his discovery of the circula- 

 tion of one sun round another, or rather of both 

 round their common centre of gravity. Herschel's 

 first list of orbital stars (Phil. Trans. 1803, where 

 this splendid discovery was first published 3 ) includes 

 the chief examples now known ; and they have all 

 been confirmed. That of which the revolution is 

 most rapid is Herculis, which has a period of 31^ 

 years, and consequently has revolved twice round 

 since it was first observed, whilst the slow planet 

 Uranus has not yet returned to the position of its 

 first discovery. Herschel does not appear to have 

 received a medal or other public recognition of this 

 signal success. 



Of the subsequent progress of this interesting in- 

 quiry we shall speak in the latter part of this chapter. 



(187.) 

 Double 

 stars or- 

 bital mo- 

 tion; 



(188.) 

 failed to 

 discover 

 parallax. 



(189.) 



1 Flamsteed appears to have entertained in 1681 a somewhat similar opinion, as we find from a letter published (1855) in 

 Sir David Brewster's Life of Newton, vol. ii., p. 103. 



2 See art. (85) and note. 



3 It was in fact distinctly though incidentally announced in his paper of 1802, though the proofs were given the following 

 year. See Phil. Trans. 1802. 



