Stellar Chemistry--Four Orders of Stars--Their Relative Ages--Gaseous Stars--Spectroscopic Star-Catalogues--Stellar Chemistry--Hydrogen Spectrum in Stars--The Draper Catalogue--Velocities of Stars in Line of Sight--Spectroscopic Binaries--Eclipses of Algol--Catalogues of Variables--New Stars--Outbursts in Nebulæ--Nova Aurigæ--Nova Persei--Gaseous Nebulæ--Variable Nebulæ--Movements of Nebulæ--Stellar and Nebular Photography--Nebulæ in the Pleiades--Photographic Star-charting--Stellar Parallax--Double Stars--Stellar Photometry--Status of Nebulæ--Photographs and Drawings of the Milky Way--Star Drift 372 CHAPTER XIII METHODS OF RESEARCH Development of Telescopic Power--Silvered Glass Reflectors--Giant Refractors--Comparison with Reflectors--The Yerkes Telescope--Atmospheric Disturbance--The Lick Observatory--Mechanical Difficulties--The Equatoreal _Coudé_--The Photographic Camera--Retrospect and Conclusion 428 APPENDIX Chronology, 1774-1893--Chemical Elements in the Sun (Rowland, 1891)--Epochs of Sun-spot Maximum and Minimum from 1610 to 1901--Movements of Sun and Stars--List of Great Telescopes--List of Observatories employed in the Construction of the Photographic Chart and Catalogue of the Heavens 443 INDEX 471 LIST OF ILLUSTRATIONS Photograph of the Great Nebula in Orion, 1883 _Frontispiece_ Photographs of Jupiter, 1879, and of Saturn, 1885 _Vignette_ Plate I. Photographs of the Solar Chromosphere and Prominences _To face p. 198_ Plate II. Photograph of the Great Comet of May, 1901 (Taken at the Royal Observatory, Cape of Good Hope) " 343 Plate III. The Great Comet of September, 1882 (Photographed at the Cape of Good Hope) " 359 Plate IV. Photographs of Swift's Comet, 1892 " 368 Plate V. Photographic and Visual Spectrum of Nova Aurigæ " 396 Plate VI. Photograph of the Milky Way in Sagittarius " 424 HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY _INTRODUCTION_