250 



COSMOS. 



principal planets, still remain to be mentioned in our list of 

 planetary bodies. Next to the actual planets and the new 

 cosmical bodies which shine forth suddenly as stars of the 

 1st magnitude, the comets, when, during their usually brief 

 appearance they are visible to the naked eye, contribute the 

 most vivid animation to the rich picture I had almost said 

 the impressive landscape of the starry heavens. 



The knowledge of the proper motion of the fixed stars 

 is closely connected historically with the progress of the 

 science of observation through the improvement of instru- 

 ments and methods. The discovery of this motion was first 

 rendered practicable when the telescope was combined with 

 graduated instruments; when from the accuracy of within a 

 minute of an arc (which after much pains Tycho Brahe first 

 succeeded in giving to his observations on the Island of Hven) 

 astronomers gradually advanced to the accuracy of a second 

 and the parts of a second ; and when it became possible to 

 compare with one another results separated by a long series of 

 years. Such a comparison was made by Halley with respect to 

 the positions of Sirius, Arcturus, and Aldebaran, as determined 

 by Ptolemy in his Hipparchian catalogue, 1844 years before. 

 By this comparison he considered himself justified (1717) in 

 announcing the fact of a proper motion in the three above- 

 named fixed stars. 2 The high and well-merited attention 

 which, long subsequent even to the observations of Flamstcad 

 and Bradley, was paid to the table of right ascensions con- 

 tained in the Triduum of Romer, stimulated Tobias Mayer 

 (1756), Maskelyne (1770), and Piazzi (1800), to compare 



2 Halley. in the Philos. Transact, for 1717-1719, vol. xxx. 

 p. 736. The essay, however, referred solely to variations 

 in latitude. Jacques Cassini was the first to add varia- 

 tions in longitude. (Arago, in the Annuaire pour 1842', 

 p. 387.) 



