xxiv Contents 



PAGE 



brightness as a test of nearness : measurement 

 of brightness : " space-penetrating " power of a 

 telescope 332 



259. Nebulae and star clusters : Herschel's great cata- 

 logues 336 



260. Relation of nebulae to star clusters: the "island 

 universe " theory of nebulae : the " shining fluid " 

 theory: distribution of nebulae. . . . 337 



261. Condensation of nebulae into clusters and stars . 339 



262. The irresolvability of the Milky Way . . . 340 



263. Double stars : their proposed employment for find- 

 ing parallax : catalogues : probable connection 

 between members of a pair . . . -341 



264. Discoveries of the revolution of double stars : 



binary stars : their uselessness for parallax . 343 



265. The motion of the sun in space : the various 



positions suggested for the apex . . . . 344 



266. Variable stars: Mira and Algol', catalogues of 

 comparative brightness : method of sequences : 

 variability of a Herculis 346 



267. Herschel's work on the solar system : new satellites : 



observations of Saturn, Jupiter, Venus, and Mars 348 



268. Observations of the sun : Wilson : theory of the 



structure of the sun 350 



269. Suggested variability of the sun . . . .351 



270. Other researches 352 



271. Comparison of Herschel with his contemporaries : 



Schroeter 352 



CHAPTER XIII. 

 THE NINETEENTH CENTURY, 272-320 . . . 354-409 



272. The three chief divisions of astronomy, observa- 

 tional, gravitational, and descriptive . . . 354 



273. The great growth of descriptive astronomy in the 



nineteenth century 355 



274. Observational Astronomy. Instrumental advances: 



the introduction of photography . . . 357 

 275. The method of least squares : Legendre and Gauss 357 



276. Other work by Gauss : the Theoria Motus : re- 



discovery of the minor planet Ceres . . 358 



