| Active Prominence of the Sun, 140,000 Miles High | Frontispiece
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| FACING PAGE |
| Nicholas Copernicus | 64 |
| Galileo Galilei | 64 |
| Johann Kepler | 65 |
| Sir Isaac Newton | 65 |
| The Hundred-Inch Reflecting Telescope at Mount Wilson | 96 |
| The Forty-Inch Refracting Telescope, Yerkes Observatory | 96 |
| 150-Foot Tower, Mount Wilson, a Diagram of Tower and Pit | 97 |
| 150-Foot Tower—Exterior View | 97 |
| View Looking Down into the Pit Beneath 150-Foot Tower | 97 |
| Mount Wilson Solar Observatory—the 100-Foot Dome | 128 |
| Mount Chimborazo, the Best Site in the World for an Observatory | 128 |
| Lick Observatory, Mount Hamilton, California | 129 |
| Photographing with the 40-inch Refractor | 129 |
| Great Sunspot Group of August 8, 1917 | 160 |
| Calcium Flocculi on the Sun | 161 |
| Eclipse of the Moon, with the Lunar Surface Visible | 161 |
| Moon's Surface in the Region of Copernicus | 192 |
| South Central Portion of the Moon, at Last Quarter | 193 |
| Corona of the Sun During an Eclipse | 224 |
| Venus, in the Crescent Phase | 225 |
| Mars, Showing Bright Polar Cap | 225 |
| Jupiter, the Giant Planet | 256 |
| Neptune and Its Satellites | 256 |
| Saturn, with Edge of Rings only in View | 257 |
| Saturn, with Rings Displayed to Fullest Extent | 257 |
| Two Views of Halley's Comet | 288 |
| Swift's Comet, which Showed Remarkable Transformations | 288 |
| Meteor Trail in Field with Fine NebulÆ | 289 |
| Ring Nebula in Lyra | 320 |
| Dumb-bell Nebula | 321 |
| Star Clouds and Black Holes in Sagittarius | 352 |
| Great Nebula in Andromeda | 353 |
