CHAP. III., 6.] ASTRONOMY. M. STRUVE SIR JOHN HERSCHE L. 



63 



6. Sidereal Astronomy since 1820. M. STRUVE Double Stars. Observatories of Dorpat and 

 Pulkowa. SIR JOHN HERSCHEL Orbits of Double Stars. Magnitudes of Stars. Variable 

 Stars. EARL OP ROSSE His Telescopes. Nebula. HENDERSON and BESSEL Parallax of 



Stars. 



(289.) As it is absolutely necessary to bring this chapter 

 Continua- to a speedy close, and as I have already anticipated, 

 So? o 1 ? in the account of Sir William Herschel's discoveries, 

 Sidereal P art f what relates to the recent history of the As- 

 Astrono- tronomy of the Fixed Stars, particularly the " Con- 

 stitution of the Heavens," and the motion of our 

 system in space, I shall condense within brief com- 

 pass a few leading facts connected, in the first place, 

 with the Orbits of Double Stars, the Brightness of 

 Stars, and the constitution of Nebulae, and these sub- 

 jects I shall connect with the names of the elder M. 

 Struve, Sir John Herschel, and Lord Rosse; the second 

 topic shall be the Parallax and distance of the fixed 

 stars, as ascertained more particularly by the late 

 professors Henderson and Bessel. 

 (290.) FRIEDRICH G-EORG WILHELM STRUVE has been the 

 M. Struve. mos t assiduous observer of double stars since the time 

 of Sir William Herschel. No discovery in this de- 

 partment can for a moment compete with the great 

 one of the orbital revolution of one star round an- 

 other. But M. Struve, by devoting his chief energies 

 during the most active years of his life, since 1813, 

 to the assiduous continuation of Herschel's observa- 

 tions, has added immensely to our knowledge of these 

 systems, and has earned the reputation of one of the 

 most skilful of modern practical astronomers. His 

 most elaborate observations were made at the Rus- 

 sian Observatory at Dorpat, with a noble refractor by 

 Fraunhofer, nearly 10 inches aperture, and 13 J feet 

 in focal length. He has published three works on the 

 subject of double stars, one in 1824, one in 1837, 

 and one in 1852, besides minor papers. The second 

 of these works contains the particulars of about 3000 

 double stars, deduced from a survey of the heavens, 

 in which at least 120,000 stars were examined. 

 (291.) M. Struve's papers are distinguished by the ela- 

 boration of the reductions, and of the statistical re- 

 double suits deduced from them. In his last publication he 

 stars. has made an interesting estimate of the number of 

 true double stars in the heavens, which, it appears, 

 is much greater in proportion to the whole number 

 than is usually believed. But it is first necessary to 

 distinguish those which are physically double from 

 those which are merely apparently or optically so. 

 The criteria on which he principally depends are 

 (1.) the fact of observed orbital revolution; but as 

 this is established in comparatively few instances, he 

 very reasonably admits (2.) a common proper motion 

 of the two components as a proof of their connection. 

 He thus finds the evidence for physical duplicity to 

 be much stronger for the closer double stars, and also 

 for brighter or nearer stars, as compared with those of 

 less magnitude. On the whole he concludes, that of 



1973 double stars, 1702, or six-sevenths of the whole, 

 are physically connected, and that, amongst at least 

 the brighter stars, the number of compound systems 

 is to that of isolated stars in a ratio not less than 1 

 to 3, perhaps even 1 to 2. 



M. Struve has also determined with extreme care .(292.) 

 the places and proper motions of the double stars ;** Struve 

 and he has formed a decided opinion, that the proper potion's!* 

 motions diminish on the whole regularly with the order 

 of magnitude, thus confirming the criterion of in- 

 creased distance from diminished brilliancy, by that 

 of the apparent displacement of the stars by the mo- 

 tion of our system. 



Besides these important works on Sidereal Astro- (293.) 

 nomy, M. Struve is well known as the head and di- Directs th e 

 rector of perhaps the best organized observatory in tQ^o" 

 the world, that of Pulkowa near St Petersburg, of Ptakowa. 

 which he has published a very interesting descrip- 

 tion. Besides other noble instruments, it contains 

 the finest refractor in Europe, that by Merz, 15 inches 

 in diameter, and 22 feet focal length* The observa- 

 tions with this noble telescope are chiefly made by his 

 son M. Otto Struve, the author of many good papers. 

 We have seen in a former section (252), that we owe 

 in great part to M. Struve the conduct of the most 

 extensive trigonometrical operation ever undertaken. 



SIR JOHN FREDERICK WILLIAM HERSCHEL, son of (294.) 

 Sir William Herschel, whilst conversant with almost fe i r J hn 



every branch of science, has devoted himself with re- Herschel 



iii i ^ . -, h is astrono- 



markable success to the cultivation of Sidereal As- m i ca i C a- 



tronomy. " Bearing a name honoured and revered reer. 

 by all, his career at Cambridge reflected upon it fresh 

 lustre ; the variety and extent of his acquirements 

 gave him a reputation amongst his college contem- 

 poraries, afterwards fully confirmed by the not more 

 impartial voice of mankind at large." He was senior 

 wrangler in 1813. " Since that time he has been 

 indefatigable as an author : first, in systematizing 

 the higher mathematics, and in forwarding their study 

 in his own university ; afterwards by treatises con- 

 tributed to the Encyclopaedia Metropolitans on Sound, 

 Light, and Physical Astronomy, which still rank 

 among the clearest, completest, and most philoso- 

 phical in our language. About the same time he 

 wrote experimental essays on different branches of 

 Chemistry, Magnetism, and Optics, and commenced 

 his purely astronomical investigations, chiefly on 

 nebulas and double stars, partly in conjunction with 

 Sir James South, of which the details are given in 

 different volumes of the Astronomical, and of the 

 Royal Society's Transactions. These memoirs collec- 

 tively include a complete revision of the objects of 

 the same description catalogued and classified by Sir 



