xxvi Contents 



PAGE 



297. The surfaces of Mars, Jupiter, and Saturn : the 



canals on Mars : Maxwell's theory of Saturn's 

 rings : the rotation of Mercury and of Venus 383 



298. The surface of the sun : Schwabe's discovery of 



the periodicity of sun-spots : connection be- 

 tween sun-spots and terrestrial magnetism: 

 Carringtorfs observations of the motion and 

 distribution of spots: Wilson's theory of spots 385 



299-300. Spectrum analysis: Newton, Wollaston, Fraun- 



ho/er, Kirchhoff : the chemistry of the sun . 386 



301. Eclipses of the sun: the corona, chromosphere, 

 and prominences : spectroscopic methods of 

 observation 389 



302. Spectroscopic method of determining motion to 

 or from the observer : Doppler's principle : 

 application to the sun 391 



33- The constitution of the sun .... 392 



304-5. Observations of comets: nucleus: theory of the 

 formation of their tails : their spectra : re- 

 lation between comets and meteors . . 393 



306-8. Sidereal astronomy : career of John Herschel : his 

 catalogues of nebulae and of double stars : 

 the expedition to the Cape : measurement of 

 the sun's heat by Herschel and by Pouillet . 396 



309. Double stars : observations by Struve and 



others : orbits of binary stars . . . 398 



310. Lord Rosse's telescopes: his observations of 

 nebulae : revival of the "island universe " 

 theory . ' 400 



311. Application of the spectroscope to nebulae: 



distinction between nebulae and clusters . 401 



312. Spectroscopic classification of stars by Secchi: 

 chemistry of stars : stars with bright-line 

 spectra 401 



313-4. Motion of stars in the line of sight. Discovery 

 of binary stars by the spectroscope : eclipse 

 theory of variable stars .... 402 



3 I S- Observations of variable stars .... 403 



316. Stellar photometry: Pogsorfs light ratio: the 



Oxford, Harvard, and Potsdam photometries 403 



317. Structure of the sidereal system: relations of 



stars and nebulae 405 



