N Naval Observatory at Washington, 122. Nebulæ, origin of, 10, 11. amorphous, 18, 44. planetary, 18. spectrum of amorphous, 45. Nebulæ, spiral, 17-25, 44. evolved from disrupted stars, 10-15. relation to novæ, 14-16. corpuscular character of, 15, 16. knots and patches of, 15. most common, 19, 20. two-armed, 20, 25. central nucleus, globular, 21. not due to explosive action, 22, 23, 25. not caused by disintegration, 24, 25. cause of development, 24, 25. spectrum of, 45-48. composed of flocks of meteorites, 48, 49. constitution established by spectroscope, 49, 50. Nebular hypotheses, 173. Neologic times, clearing of sky in, 185. Neptune, 118. rotates backward, 118. owes discovery to mathematical triumph, 119-126. faint belts on, 168. further advanced than giant planets, 168. Newcomb, 67. Newton, Professor, 36, 42. Newton, Sir Isaac, 34. Nova Aurigæ, 7, 8, 12. history chronicled by its spectrum, 8, 9. Nova Cygni, 7. Novæ, 6, 7. origin 5, 10. first chronicled, 5. spectroscopic study of, 7. Nova Persei, 7. history of, 12-15. O Oceans, none on Mars, 91. evaporation of, 204. basins of, on Moon, 204-208. basins of, on Mars, 206, 207. Olbers, 97. Olmstead, Professor, 33. Orbital distance, of Mercury, 62. of Venus, 73. of Mars, 91. of Eros, 94. of Saturn, 108. Orbital tilts, of asteroids, erratic, 100, 101. of satellites of Uranus, 116. of planets, substantially the same, 129-131, Notes 244. deviation from rule, by Mercury, 131. of satellites, increase with distance from primary, 133, 134. Orbits, determining factors, 35. rendered more circular by collisions, 141-143, Notes 250, 251. made more conformant to general plane by collisions, 141-143. Orion, great nebula in, 18.