RESISTING MEDIUM. 47 



through a gaseous or ethereal fluid loses only -g^th of its in- 

 tensity, this assumption, which gives the amount of the 

 density of a fluid capable of diminishing light, would suffice to 

 explain the phenomena as they manifest themselves. Among 

 the doubts advanced by the celebrated author of "The New 

 Outlines of Astronomy," against the views of Olbers and 

 Struve, one of the most important is that his twenty-feet 

 telescope shows, throughout the greater portion of the Milky 

 Way in both hemispheres, the smallest stars projected on a 

 black ground. 35 



A better proof, and one based, as we have already stated, 

 upon direct observation of the existence of a resisting fluid, 36 

 is afforded by Encke's comet, and by the ingenious and im- 

 portant conclusion to which my friend was led in his observa- 

 tions on this body. This resisting medium, must, however, be 

 regarded as different from the all-penetrating light-ether, be- 

 cause the former is only capable of offering resistance inasmuch 

 as it cannot penetrate through solid matter. These observa- 

 tions require the assumption of a tangential force to explain the 

 diminished period of revolution (the diminished major-axis of 

 the ellipse), and a this is most directly afforded by the hypothesis 

 of a resisting fluid. 37 The greatest action is manifested during 



35 "Throughout by far the larger portion of the extent of the 

 Milky Way in both hemispheres, the general blackness of the 

 ground of the heavens, on which its stars are projected .... 

 In those regions where the zone is clearly resolved into stars, 

 well separated, and seen projected on a black ground., and 



where we look out beyond them into space " Sir 



John Herschel, Outlines ofAstr., pp. 537, 539'. 



16 Cosmos, vol. i. pp. 69, 70, 92; compare also Laplace, 

 Essai Philosophique sur les Probability, 1825, p. 133 ; Arago 

 in the Annuaire du Bureau des Long, pour 1832, p. 188 

 pour 1836, p. 216 ; and Sir John Herschel, Outlines of Astrl, 

 577. 



7 The oscillatory movement of the emanations from the 

 head of some comets, as in that of 1 744, and in Halley's as 



