DOUBLE STARS 205 



dation of this important branch of astronomy. In 1820, 

 Struve published his first Dorpat Tabic of double stars, 790 

 in number. This was followed in 1824 by a second, con- 

 taining 3112 double stars, down to the ninth magnitude, in 

 distances under 32", of which ordy about one sixth had been 

 before observed. To accomplish this work, nearly 120,000 

 fixed stars were examined by means of the great Fraun- 

 liofer refractor. Struve's third table of multiple stars ap- 

 peared in the year 1837, and forms the important work Stel- 

 lanim comjyositarum Mcnsurce Miwometriccc.^ It contains 

 2787 double stars, several imperfectly observed objects being 

 carefully excluded. 



Sir John Herschel's unwearied diligence, during his four 

 years' residence in Feldhauscn, at the Cape of Good Hope, 

 which, by contributing to an accurate topograpliical knowl- 

 edge of the southern hemisphere, constitutes an epoch in 

 astronomy,! has been the means of enriching this number 

 by the addition of more than 2100 double stars (which, with 

 few exceptions, had never before been observed). All these 

 African observations were taken by a twenty-feet reflecting 

 telescope ; they were reduced for the year 1830, and are 

 included in the six catalogues which contain 3346 double 

 stars, and were transmitted by Sir John Herschel to the As- 

 tronomical Society for the sixth and ninth parts of their val- 

 uable Memoirs.X In these European catalogues are laid 

 down the 380 double stars which the above celebrated as- 

 ironomer had observed in 1825, conjointly with Sir James 

 South. 



We trace in this historical sketch the gradual advance 

 made by the science of astronomy toward a thorough knowl- 

 edge of _^;ar?mZ, and especially oi binary systems. The num- 

 ber of double stars (those both optically and physically double) 

 may at present be estimated with some certainty at about 

 6000, if we include in our calculation those observed by Bes- 

 sel with the excellent Fraunhofer heliometer, by Argelan- 

 der§ at Abo (1827-1835), by Encke and G-alle at Berlin 



* Stnive, Mensurce Microm., p. 40 and i234-248. On the whole, 

 2G41-^14G, i. e., 2787 double stars have been observed. (JMiidLer, in 

 Schum., Jahrb., 1839, s. 64.) 



t Sir Johu Herschel, Astron. Ohserv. at the Cape of Good Hope, p. 

 165-303. _ _ X Ihid.,^. IQ7 ixml 2^2. 



$ Arselander, in order carefiilly to investigate their [)roper motion, 

 examined a great number of fixed stars. See his essay, entitled ''DLX. 

 StcUarum jixarnm positiones medics, ineunfe anno 1830, ex ohserv. Abacs 

 habitis {HelsingforsicB, ISS.'S)." Madler {Asi?\, s. 62.5) estimates the 



