UESISTlNCi MKDIUM. 39 



stars,* and iVom his ingenious experiments on the spaee-pen- 

 etratinir 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 ^^th of its intensity, this assump- 

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

 ble of diminishing light, would sulhce to explain the ]ihe- 

 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 Struve, one 

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

 throughout 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 liuid,| 

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

 portant conclusion to whicli my friend was led in his observ- 

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

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

 because the former is only capable of ofTering 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 afibrded 

 by the hypothesis of a resisting fluid. s^ The greatest action 



* Cosmos, vol. i., p. 86, 87. 



t "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 i-esolved into stars, well separated, and seen pi'ojected 

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

 ■ — Sir John Herschel, Outlines of Astr., p. 537, .539. 



X Cosmos, vol. i., p. 85, 86, 107 ; compare also Laplace, Essai Philos- 

 ophique sur les Prohabilit.es, 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 Asir., § d77. 



§ The oscillatory movement of the emanations from the head of some 

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

 tween the 12th and 22d of October, ]835 (Schumacher, Astrou. 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 the ordinary attracting force of the sun ;" 

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

 comet (whose period of revolution is 3 J years), can not be regarded as 

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

 portant subject, Bessel. in Sclium., Astron. Nachr., No. 289, s. 6, and 

 No. 310, s. 345-350, with Encke's Treatise on the hypothesis of the re« 

 sistim; medium, in Schum., No. 305. s. 265-274 



