RESISTING MEDIUM. 39 



Btars,^ and iVom his ingenious experimeu.ts on the space-pen- 

 etrating power of his great telescopes, seem to show, that if 

 the light of Sirius in its passage to us through a gaseous or 

 ethereal fluid loses only -g- ^Q-th of its intensity, this assump- 

 tion, which gives the amount of the density of a fluid capa- 

 ble of diminishing light, would suffice to explain the phe- 

 nomena as they manifest themselves. Among the doubts 

 advanced by the celebrated author of "The New Outlines 

 of Astronomy" against the views of Olbers and Struvc, one 

 of the most important is that his twenty-feet telescope shows, 

 tliroughout the greater portion of the Milky Way in both hem- 

 ispheres, the smallest stars projected on a black ground. f 



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

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

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

 portant conclusion to which my friend Avas led in his observ- 

 ations on this body. This resisting medium miist, however, 

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

 because the former is only capable of ofiering resistance in- 

 asmuch as it can not penetrate through solid matter. These 

 observations require the assumption of a tangential force to 

 explain the diminished period of revolution (the diminished 

 major axis of the ellipse), and this is most directly afiorded 

 by the hypothesis of a resisting fluid. § The greatest action 



* Coamos, vol. i., p. SG, 87. 



T "Throughout by far ihe larger portion of the extent of the Milky 

 Way ill botli hemispheres, the general blackness (jf the ground of the 

 heavens, on which its stars are projected .... In tliose regions where 

 the zone is clearly resolved into stars, well separated, and seen projected 

 on a hind: ground, and where we look out beyond them into space. . . ." 

 — Sir John Herschel, Outlines of As/'r., p. .537, 539. 



+ Cosmos, vol. i,, p. 8"), 86, 107; compare also Laplace, Essai Philos- 

 ophique sur les ProbabilUes, 1825, p. 133; Arago, in the Anmiaire d?i 

 Bureau des Long, pour 1832, p. 188, pour 183G, \>. 216; and Sir John 

 Herschel, Outlines of Astr., § 577. 



^ The oscillatory movement of the emanations from the bead of some 

 comets, as in tliat of 1744, and in Halley's, as observed by Bessel, be- 

 tween the 12th and 22d of October, 1835 (Schumacher, Astron. Nachr., 

 Nos. 300, 302, § 185, 232), "may indeed, in the case of some individ- 

 uals of this class of cosmical bodies, exert an influence on the transla- 

 tory and rotatory motion, and lead us to infer the action of polar forces 

 ("$ 201, 229), which differ from theordinary attracting force of the sun;'' 

 but the regular acceleration observable for sixty-three years in Encke's 

 pomet (whose period of revolution is 3^ years), can not be regarded as 

 the result of incidental emanations. Compare, on this cosmically im- 

 portant subject, Bessel, in Schum.. Astron. Nachr., No. 289, s. G, and 

 No. 310, s. 345-350, with Encke's Treatise on the hypothesis of Iho rf> 

 slstliiic mc'dium, in Schum., No. 305. s. 285-274 



