§ 302. Spectroscopic method of determining motion to or from the observer: _Doppler’s principle_: application to the sun 391 § 303. The constitution of the sun 392 §§ 304-5. Observations of comets: _nucleus_: theory of the formation of their tails: their spectra: relation 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 § 315. Observations of variable stars 403 § 316. Stellar _photometry_: _Pogson’s_ light ratio: the Oxford, Harvard, and Potsdam photometries 403 § 317. Structure of the sidereal system: relations of stars and nebulae 405 §§ 318-20. Laplace’s nebular hypothesis in the light of later discoveries: the sun’s heat: _Helmholtz’s_ shrinkage theory. Influence of tidal friction on the development of the solar system: Professor Darwin’s theory of the birth of the moon. Summary 406 LIST OF AUTHORITIES AND OF BOOKS FOR STUDENTS 411 INDEX OF NAMES 417 GENERAL INDEX 425 LIST OF ILLUSTRATIONS. FIG. PAGE The moon _Frontispiece_ 1. The celestial sphere 5 2. The daily paths of circumpolar stars _To face p._ 8 3. The circles of the celestial sphere 9 4. The equator and the ecliptic 11 5. The Great Bear _To face p._ 12 6. The apparent path of Jupiter 16