SIDEREAL ASTRONOMY. 329 



the worlds which surround us. Modern science is thoroughly 

 awake to all these points of higli philosophic enquiry. 



While upon this topic we must briefly advert to the seem- 

 ingly more direct evidence of an ether, derived from the 

 diminishing periods of Encke's comet, and from the zodiacal 

 light. Upon the latter proof we cannot at present place any 

 certain reliance. But the observations on Encke's comet 

 have high value from their uniformity of result ; showing a 

 tangential force acting constantly upon this body in its orbit, 

 which can only well be explained by the supposition of its 

 motion through a resisting medium. Here again time, and 

 repeated observations, will probably give certainty to our 

 future knowledge. 



The second chapter, ' on Natural and Telescopic Vision, 

 the Velocity of Light, and Photometry,' is associated with 

 a valuable note by Arago, upon the effects of telescopic 

 glasses on the visibility of the fixed stars. He conceives that 

 high magnifying powers facilitate the finding of any star, not 

 by sensibly enlarging its image, but by bringing to the eye a 

 larger quantity of light, and at the same time contrasting it 

 more strongly with the aerial field through which the star is 

 seen ; the telescope magnifying, according to his view, the 

 distance between the illuminated particles of air in the tele- 

 scopic area surrounding the star, and thus giving a darker 

 surface in contrast with the intense and concentrated light of 

 the latter. There may perhaps be a little fancy in the latter 

 part of this explanation ; but we are not entitled to cavil at 

 it, having before us the many extraordinary and complex 

 phenomena of the visual power as directed, with or without 

 aid, into these regions of space. We have always considered 

 the original papers of Sir W. Herschel on this subject, the elo- 

 quent commentary upon them by Arago,* and the labours of 



* Analyse Historique et Critique de la Vie et des Travaux de Sir William 

 Herschel. Annuaire pour V An 1842, par le Bureau des Longitudes; a memoir 



