MILKY WAY. 149 



of Cepheus, and therefore near Cassiopeia (from which con- 

 stellation we began our description of the Milky Way), to- 

 ward Ursa Minor and the pole. 



From the extraordinary advancement which the applica- 

 tion of large telescopes has gradually effected in our know! 

 edge of the sidereal contents and of the differences in the 

 concentration of light observable, in individual portions of the 

 Milky Way, views of merely optical projection have been re- 

 placed by others referring rather to physical conformation. 

 Thomas Wright, of Durham, # Kant, Lambert, and at first 

 also Sir William Herschel, were disposed to consider the 

 form of the Wilky Way, and the apparent accumulation of 

 the stars within this zone, as a consequence of the flattened 

 form and unequal dimensions of the world-island (starry 

 stratum) in which our solar system is included. The hy- 

 pothesis of the uniform magnitude and distribution of the 

 fixed stars has recently been attacked on many sides. The 

 bold and gifted investigator of the heavens, Wm. Herschel, 

 in his last works,! expressed himself strongly in favor of the 

 assumption of an annulus of stars ; a view which he had 

 contested in the talented treatise he composed in 1784. The 

 most recent observations have favored the hypothesis of a 

 system of separate concentric rings. The thickness of these 

 rings seems very unequal ; and the different strata, whose 

 combined stronger or fainter light we receive, are undoubt- 

 edly situated at very different altitudes, i. e., at very unequal 

 distances from us ; but the relative brightness of the sep- 

 arate stars which we estimate as of the tenth to the six- 

 teenth magnitude, can not be regarded as affording sufficient 

 data to enable us in a satisfactory manner to deduce numer- 

 ically from them the radius of their spheres of distances. $ 



In many parts of the Milky Way, the space-penetrating 

 power of instruments is sufficient to resolve whole star- 

 clouds, and to show the separate luminous points projected 

 on the dark, starless ground of the heavens. We here act- 



* De Morgan has given an extract of the extremely rare work of 

 Thomas Wright of Durham ( Theory of the Universe, London, 1750), p 

 2-11 in the Philos. Magazine, ser. iii., No. 32. Thomas Wright, to whose 

 researches the attention of astronomers has been so permanently di 

 rected since the beginning of the present century, through the ingen 

 ions speculations of Kant and William Herschel, observed only with a 

 reflector of one foot focal length. 



t Pfaff, in Will. HerscheVs sammtl. Schriften, bd. i. (1826), s. 78-8i { 

 Struve, Etudes StelL, p. 35-44. 



t Encke, in Schumacher's Astr. Nachr., No. 622, 1817 «» 3U-3ie 



