80 Professor Struve on Double and Triple Stars, as 



other, or at least that they are so close to each other that they 

 cannot be separated by the best glasses. Two stars of the 

 third magnitude are in this respect the most remarkable, name- 

 ly, £ Hercules and 3 Cygni. All attempts which have been 

 lately made to see these satellite stars have failed, y Virgi- 

 nis is at present a double star of the first class, whilst Her- 

 schel had reckoned it one of the third. Other stars which were 

 single have become double: Thus £ Orionis is now a double 

 star of the first class, which is easily recognized, while Herschel 

 had decidedly seen it single. This phenomenon is explained 

 by the slowness of the apparent revolution of the satellite star. 

 Herschel has also directed the attention of astronomers to the 

 difference of colour in the double stars ; but many persons 

 look upon this difference as depending upon the observer. 

 The new observations made in England and at Dorpat have 

 fully corroborated those of Herschel, in showing that the bright 

 star is very often yellow, while its companion is blue or violet. 

 It was in the year 1824 that the great achromatic telescope 

 of Fraunhofer arrived at the Observatory of Dorpat. It is a 

 monument of the progress of astronomy in Russia. Four 

 observatories completely furnished with instruments have been 

 founded within the last twenty years in this empire ; namely, 

 at Dorpat, Abo, Warsaw, and Nicolajef ; so that there are at 

 present more establishments of this kind in Russia than in 

 any other kingdom in Europe. To make use of the great 

 telescope of Fraunhofer in prosecuting farther inquiries into 

 the double stars was an employment not unworthy of this 

 noble instrument. The new measurements made in England 

 and at Dorpat already exceeded in exactness those of the great 

 astronomer who discovered Uranus, owing to the improve- 

 ment in micrometers ; but the achromatic telescopes made use 

 of were well known to be inferior in an optical point of view 

 to the reflecting telescope of Herschel. A telescope, therefore, 

 which could in this respect stand a comparison with these in- 

 struments, and which had a decided superiority in its micro- 

 metric apparatus, was well fitted to give a greater extension 

 to preceding discoveries. What appeared to me most important 

 was to attempt with Fraunhofer , s telescope to make a survey 

 of all the stars of a certain brightness in the part of the Heavens 



