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V}8 COSMOS.*tra jijtxLof Manilius, which corresponds with our term fixed6tars> This idea of fixity leads to the secondary idea ofimmobiHty, of persistence in one spot, and thus the originalsignification of the expressions injixum or ajjixum sidus waagradually lost sight of in the Latin translations of the MiddleAges, and the idea of immohihty alone retained. Thisis already apparent in a highly rhetorical passage of Seneca,regarding the possibility of discovering new planets, in which" Credis autem in hoc max-he says (JXcit. Quccst., vii., 24),imo et pulcherrimo corpore inter innumerabiles stcllas, qua?nocteiTi decore vario distinguunt, qua3 aera minime vacuumet mertem esse patiuntur, quinque solas esse, quibus exercerese liceat ;ceteras stare Jixuni ct iimnohilcm 'po'puliunV'"And dost thou believe that in this so great and splendidbody, among innumerable stars, which by their various beautyadorn the night, not sufiering the air to remain void andfive stars to whom itunprofitable, that there should be onlyispermitted to be in motion, while all the rest remain a fixedand immovable multitude ?" This fixed and immovable mul*titude is nowhere to be found.In order the better to classify the main results of actualobservations, and the conclusions or conjectures to whichthey give rise, in the descrij)tion of the universe, I will separatethe astrognostic sphere into the following sections :I. The considerations on the realms of space aiid the bodiesby which they appear to be filled.II. Natural and telescopic vision, the scintillation of thestars, the velocity of light, and the photometric experimentson the intensity of stellar light.III. The number, distribution, and color of the stars ;thestellar swarms, and the Milky Way, which is interspersedwith 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 problematij3alexistence of dark cosmical bodies ;the parallax and measureddistance of some of the fixed stars.VI . The double stars, and the period of their revolutionround a common center of gravity.VII. The nebulse which are interspersed in the Magellaniaclouds with numerous stellar mass(js, the black spots (coallags) in the vault of heaven.• Cicei o, X>eiVa^ /Vorvr i. •, J3 ; Pliii., ii. 6 and 21 ; Mauillas, ii., 35