The object of the present work is not so much to instruct as to suggest. It represents a sort of reconnaissance, and embodies the information collected by scouts and skirmishers regarding practicable lines of advance and accessible points of attack, with a view to annexing for the realm of knowledge some further strips and corners from the territory of ignorance. Its inspiring motive, in short, is the desire for a rectification of the frontier in the interests of science. Material resources for the furtherance of such encroachments are not lacking. The globe is studded with observatories, variously and admirably equipped. Yet innumerable objects in the sidereal heavens remain neglected, mainly through inadvertence to the extraordinary interest of the questions pending with respect to them. In the following pages it has been sought to indicate some of these individually, and in their relations to the larger meanings of cosmical research. But this could be done effectually only from the vantage-ground of our actual acquirements; hence the book, although primarily designed to stimulate the progress of astrophysics, necessarily includes an account of its present state. Before attempting to add to our store of learning, we must realise what is already possessed.

The unknown, it is true, is indefinitely vast, and the rays of light which we can project into its darkness penetrate but a short way. Our programme of inquiry must accordingly be limited to what is now practicable, or promises to become so in the immediate future. The unforeseen, too, will have something, perhaps much, to do with prescribing directions for fresh researches. Queries, in the coming years, will be put to the skies very different from those here propounded; and answers of a surprising kind will doubtless be afforded to our present interrogatory. The keen delight of such revelations will reward those who, loving truth for its own sake, have laboured for its promotion; and if these pages should, in any degree, help to quicken and guide their noble enthusiasm, they will have amply served their purpose.

They do not, however, cover the whole field of astrophysics. Planetary and cometary astronomy are deliberately, although for different reasons, excluded from treatment in them. The moon and planets still belong to the theoretical and descriptive departments of the elder celestial science. Nearly all that is known about their condition has been learned by direct telescopic observation. The items of information added through the aid of the camera and the spectroscope, though valuable, are very few. The orbital characteristics, on the other hand, of comets and meteors are too prominent to be set aside in any profitable discussion of their nature. They are of the very essence of the phenomena; yet they would be out of place in a book strictly limited to the consideration of the heavenly bodies under their physical aspect.

The writer has received much courteous help from various quarters in preparing the illustrations, and desires in particular to acknowledge her obligations to Sir William and Lady Huggins, to Sir David Gill, to Dr. Roberts, F.R.S., to the Rev. W. Sidgreaves, S.J., Mr. W. E. Wilson, F.R.S., M. Deslandres, Professor Hale, Professor Barnard, to Professors E. C. and W. H. Pickering, and to Professor Campbell.

LONDON, _December 1902_.

1. PROGRESS OF SIDEREAL PHYSICS 171 2. THE CLASSIFICATION OF STELLAR SPECTRA 179 3. HELIUM STARS 189 4. HYDROGEN STARS 197 5. SOLAR STARS 203 6. STARS WITH FLUTED SPECTRA 209 7. CARBON STARS 215 8. STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES 222 9. HELIUM STARS WITH BRIGHT LINES 229 10. WOLF-RAYET STARS 237 11. THE GENERAL QUESTION OF BRIGHT LINES IN STELLAR SPECTRA 243 12. ANOMALOUS AND VARIABLE SPECTRA 246 13. COLOUR VARIABILITY 253 14. THE SPECTRA OF DOUBLE STARS 263 15. THE EVOLUTION OF THE STARS 271 16. ROTATION OF THE STARS 280 17. SPECTROSCOPIC BINARIES 286 18. ECLIPSING STARS 299 19. SHORT-PERIOD VARIABLES 319 20. SHORT-PERIOD VARIABLES—_continued_ 328 21. THE PROBLEM OF BETA LYRÆ 337 22. STARS VARIABLE IN LONG PERIODS 347 23. PECULIAR AND IRREGULAR VARIABLES 363 24. TEMPORARY STARS 375 25. DARK STARS 399 26. THE GENERAL QUESTION OF STELLAR VARIABILITY 404 27. IRREGULAR STAR CLUSTERS 409 28. NEBULOUS CLUSTERS—THE PLEIADES 415 29. NEBULOUS CLUSTERS—_continued_ 423 30. GLOBULAR CLUSTERS 428 31. WHITE NEBULÆ 439 32. DOUBLE NEBULÆ 452 33. NEBULOUS STARS 460 34. PLANETARY NEBULÆ 469 35. ANNULAR NEBULÆ 484 36. THE ORION NEBULA 495 37. OTHER IRREGULAR NEBULÆ 506 38. NONDESCRIPT NEBULÆ 516 39. VARIABLE NEBULÆ 522 40. THE NATURE OF NEBULÆ 531 41. THE PHYSICS OF THE MILKY WAY 538 APPENDIX 547 INDEX 553

LIST OF ILLUSTRATIONS

PRINTED SEPARATELY PLATE I. Photograph of the Argo Nebula _Frontispiece_ II. Photograph of A-Band in Solar Spectrum (McClean) _to face page_ 22 III. Flash and Cusp Spectra compared „ 46 IV. The Sun portrayed in Calcium Light (1); Photograph of the Chromosphere, 31st May 1894 (2) „ 100 V. Types of the Corona (Hansky) „ 128 VI. The Corona of 1900 (Jewell) (1); Reversal and Distortion of the C-line (Hale) (2) „ 96 VII. The Corona of 1896 (Morin) (1); the Corona of 1898 (Wesley) (2) „ 134 VIII. The Corona of 1900 (Wesley) „ 136 IX. Photographs of Typical Star-Spectra (Huggins) „ 180 X. Photographed Spectrum of ε Canis Majoris (Lunt) (1); Photographed Spectrum of α_{2} Centauri (Gill) (2) „ 182 XI. Spectrum of α Herculis (Espin) (1); Photographed Spectrum of 152 Schjellerup (Hale) (2) „ 184 XII. Photographed Spectra of Carbon Stars (Hale and Ellerman) „ 186 XIII. Photographed Spectra of Mira and of α Herculis (Sidgreaves) „ 188 XIV. Spectrum of γ Cassiopeiæ. Photographed by the Rev. W. Sidgreaves „ 190 XV. Photographed Spectra of Types II., III., and IV. compared „ 220 XVI. Sections of the Spectra of ß Lyræ and Nova Persei „ 344 XVII. Visible Spectrum of Nova Aurigæ (Campbell) „ 376 XVIII. Nebulosity round Nova Persei (Ritchey) (1); Spectrum of Nova Persei, 11th August 1901, with corresponding Intensity-Curve (2) „ 394 XIX. Photograph of Messier 11 (Roberts) „ 410 XX. Photograph of a Spiral Nebula in Cetus (Roberts) „ 442 XXI. Photograph and Drawings of a Spiral Nebula in Pegasus (Keeler) „ 444 XXII. Photograph of Whirlpool Nebula (W. E. Wilson) „ 446 XXIII. Fan Nebulæ (Barnard) (1); Planetary Nebulæ with their Spectra (Keeler) (2, 3, 4) „ 448 XXIV. Photograph of Owl Nebula (Roberts) (1); Photograph of the Orion Trapezium (Pickering) (2) „ 472 XXV. Photograph of the Ring Nebula in Lyra (W. E. Wilson) „ 484 XXVI. Photograph of the Orion Nebula (Pickering) „ 494 XXVII. Nebulous Formation in Orion. Photographed by W. H. Pickering „ 496 XXVIII. Photograph of the Trifid Nebula (Roberts) (1); Photograph of Messier 77 (Roberts) (2) „ 510 XXIX. Photograph of a Nebula in Perseus (Barnard) „ 514 XXX. Photograph of the Dumb-Bell Nebula (W. E. Wilson) „ 516 XXXI. Photograph of the Milky Way in Ophiuchus (Barnard) „ 540

PRINTED IN THE TEXT FIG. PAGE 1. General View of the Atmospheric Spectrum 24 2. The Infra-Red Spectrum (Langley) 24 3. Oxygen-Triplet in the Solar Spectrum 29 4. Curves representing the Distribution of Light in Fraunhofer and Arc Lines (Jewell) 41 5. Diagram of the Helium Spectrum 57 6. Sun-spot photographed by Janssen, 1st April 1894 73 7. Photograph of a Bridged Sun-spot (Janssen) 76 8. Sun-spot drawn by J. de M. Pereira, 18th June 1894 78 9. Group of Sun-spots drawn by Miss E. Brown 78 10. Portion of a Sun-spot Spectrum photographed by Professor Young 89 11. Reversal of the D-Lines in the Spectrum of a Sun-spot 94 12. Prominence observed at Kalocsa, 19th September 1893 104 13. Prominence observed at Kalocsa, 3rd October 1892 107 14. Forms of a Prominence in Hydrogen, Helium, and Pseudo-Coronium (Fényi) 117 15. Dark Markings in the Corona of 1871 (Wesley) 134 16. Dark Markings in the Corona of 1896 (Wesley) 134 17. Spoerer’s Curves of Sun-spot Latitude 152 18. Curves of Sun-spot Frequency and Magnetic Agitation (Ellis) 155 19. Intensity-Curves of Ηγ in Spectrum of Mira (Campbell) 225 20. Hydrogen-Envelope of a Wolf-Rayet Star (Campbell) 241 21. Photometric Curves of Algol Variables (Pickering) 309 22. Light-Curve of η Aquilæ (Schur) 322 23. Orbit of η Aquilæ (Wright) 323 24. Light-Curve of S Sagittæ (Yendell) 326 25. Light and Velocity Curves of ζ Geminorum (Campbell) 329 26. Orbit of ζ Geminorum (Campbell) 330 27. Photometric Curve of U Pegasi (Pickering) 332 28. Light-Curves of Cluster-Variables (Bailey) 336 29. Light-Curve of β Lyræ (Argelander) 337 30. System of β Lyræ (Myers) 341 31. Distribution of the Periods of 208 Variables 347 32. Light-Curve of U Orionis (Porro) 351 33. Light-Curve of S Ursæ Majoris in 1894 354 34. Mean Light-Curve of S Ursæ Majoris 355 35. Mean Light-Curve of T Ursæ Majoris 355 36. Mean Light-Curve of T Cassiopeiæ 355 37. Light-Curve of T Andromedæ 357 38. Light-Curve of S Cephei 359 39. Light-Curve of R Scuti 364 40. Long and Short Maxima of U Geminorum 365 41. Light-Curve of S^2 Cygni 367 42. Light-Curve of Nova Aurigæ 380 43. Photographs before and after the Apparition of Nova Persei 389 44. Light-Curve of Nova Persei, 22nd February to 6th March 391 45. Light-Curve of Nova Persei, 17th March to 3rd May 1901 391 46. Exterior Nebulosities of the Pleiades 419 47. Photograph of ω Centauri (Bailey) 430 48. Drawing of a Pair of Spindle Nebulæ (Spitaler) 457 49. Sketch of T Tauri and Hind’s Nebula (Barnard) 524 50. Drawing from Photographs of T Tauri and Hind’s Nebula (Keeler) 525