V}8 COSMOS. 



* 



tra jijtxL of Manilius, which corresponds with our term fixed 

 6tars> This idea of fixity leads to the secondary idea of 

 immobiHty, of persistence in one spot, and thus the original 

 signification of the expressions injixum or ajjixum sidus waa 

 gradually lost sight of in the Latin translations of the Mid- 

 dle Ages, and the idea of immohihty alone retained. This 

 is already apparent in a highly rhetorical passage of Seneca, 

 regarding the possibility of discovering new planets, in which 

 he says (JXcit. Quccst., vii., 24), " Credis autem in hoc max- 

 imo et pulcherrimo corpore inter innumerabiles stcllas, qua? 

 nocteiTi decore vario distinguunt, qua3 aera minime vacuum 

 et mertem esse patiuntur, quinque solas esse, quibus exer- 

 cere se liceat ; ceteras stare Jixuni ct iimnohilcm 'po'puliunV' 

 "And dost thou believe that in this so great and splendid 

 body, among innumerable stars, which by their various beau- 

 ty adorn the night, not sufiering the air to remain void and 

 unprofitable, that there should be only five stars to whom it 

 is permitted to be in motion, while all the rest remain a fixed 

 and immovable multitude ?" This fixed and immovable mul* 

 titude is nowhere to be found. 



In order the better to classify the main results of actual 

 observations, and the conclusions or conjectures to which 

 they give rise, in the descrij)tion of the universe, I will sep- 

 arate the astrognostic sphere into the following sections : 



I. The considerations on the realms of space aiid the bodies 

 by which they appear to be filled. 



II. Natural and telescopic vision, the scintillation of the 

 stars, the velocity of light, and the photometric experiments 

 on the intensity of stellar light. 



III. The number, distribution, and color of the stars ; the 

 stellar swarms, and the Milky Way, which is interspersed 

 with a few nebulse. 



lY. The newly-appeared and periodically-changing stars, 

 and those that have disappeared. 



V. The proper motion of the fixed stars ; the problematij3al 

 existence of dark cosmical bodies ; the parallax and meas- 

 ured distance of some of the fixed stars. 



VI . The double stars, and the period of their revolution 

 round a common center of gravity. 



VII. The nebulse which are interspersed in the Magellania 

 clouds with numerous stellar mass(js, the black spots (coal 

 lags) in the vault of heaven. 



• Cicei o, X>eiVa^ /Vorvr •, i. J3 ; Pliii., ii. 6 and 21 ; Mauillas, ii., 35 



