tra fixa of Manilius, which corresponds with our term fixed 

 stars.* This idea of fixity leads to the secondary idea of 

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

 signification of the expressions infixum or affixum sidus was 

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

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

 is already apparent in a highly rhetorical passage of Seneca, 

 regarding the possibility of discovering new planets, in which 

 he says (Nat. Queest., vii., 24), " Credis autem in hoc max- 

 imo et pulcherrimo corpore inter innumerabiles Stellas, quae 

 noctem decore vario distinguunt, quse ae'ra minime vacuum 

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

 cere se liceat ; ceteras stare fixum et immobilempopulum?" 

 "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 suffering 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 description of the universe, I will sep- 

 arate the astrognostic sphere into the following sections : 



I. The considerations on the realms of space and 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 nebulae. 



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

 and those that have disappeared. 



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

 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 nebulas which are interspersed in the Magellanic 

 clouds with numerous stellar masses, the black spots (coal 

 bags) in the vault of heaven. 



* Cicero, De Nat Deorutn, i., 13 ; Plin., ii., 6 and 24 ; Manillas, ii., 35 



