XXV. Great Comet. Photographed May 5, 1901, with the 13-inch Astrographic Refractor of the Royal Observatory, Cape of Good Hope 210 XXVI. Photographs of Swift's Comet. By Professor E. E. Barnard 220 XXVII. Region of the Milky Way in Sagittarius, showing a Double Black Aperture. Photographed by Professor E. E. Barnard 232 XXVIII. Irregular Star Clusters. Photographed by E. E. Barnard 256 XXIX. Cluster M. 13 Herculis. Photographed by Mr. W. E. Wilson 258 XXX. Photograph of the Orion Nebula (W. H. Pickering) 262 XXXI. Photographs of Spiral Nebulæ. By Dr. Max Wolf 264 XXXII. Photograph of Whirlpool Nebula (M. 51). Taken by Mr. W. E. Wilson, March 6, 1897 266 LIST OF ILLUSTRATIONS PRINTED IN THE TEXT FIG. PAGE 1. Principle of Galilean Telescope 3 2. Principle of Common Refractor 3 3. Dorpat Refractor 7 4. Thirty-inch Refractor, Pulkowa Observatory 9 5. Principle of Newtonian Reflector 11 6. Lord Rosse's Telescope 12 7. Herschel's 4-foot Reflector 13 8. Star--Correct and Incorrect Adjustment 21 9. Small Telescope on Pillar and Claw Stand 26 10. Telescope on Tripod, with Finder and Slow Motions 27 11. Equatorial Mounting for Small Telescope 29 12. Eight-inch Refractor on Equatorial Mounting 32 13. Four-foot Reflector, equatorially mounted 36 14. Drawing of Sunspot 52 15. " " 53 16. " " 56 17. " " 57 18. " " 58 19. Eclipses of the Sun and Moon 69 20. Mercury as a Morning Star. W. F. Denning, 10-inch Reflector 84 21. The Tides 101 22. Lunar Craters 105 23. " " 118 24. Mars 146 25. Jupiter 157 26.