FIG. 1. The Constellation of Orion (Hubble) 2. The Great Nebula in Orion (Pease) 3. Model by Ellerman of summit of Mount Wilson, showing the observatory buildings among the trees and bushes 4. The 100-inch Hooker telescope 5. Erecting the polar axis of the 100-inch telescope 6. Lowest section of tube of 100-inch telescope, ready to leave Pasadena for Mount Wilson 7. Section of a steel girder for dome covering the 100-inch telescope, on its way up Mount Wilson 8. Erecting the steel building and revolving dome that cover the Hooker telescope 9. Building and revolving dome, 100 feet in diameter, covering the 100-inch Hooker telescope 10. One-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lower end of telescope tube. (Seen above) 11. The driving-clock and worm-gear that cause the 100-inch Hooker telescope to follow the stars 12. Large irregular nebula and star cluster in Sagittarius (Duncan) 13. Faint spiral nebula in the constellation of the Hunting Dogs (Pease) 14. Spiral nebula in Andromeda, seen edge on (Ritchey) 15. Photograph of the moon made on September 15, 1919, with the 100-inch Hooker telescope (Pease) 16. Photograph of the moon made on September 15, 1919, with the 100-inch Hooker telescope (Pease) 17. Hubble's Variable Nebula. One of the few nebulae known to vary in brightness and form 18. Ring Nebula in Lyra, photographed with the 60-inch (Ritchey) and 100-inch (Duncan) telescopes 19. Gaseous prominence at the sun's limb, 140,000 miles high (Ellerman) 20. The sun, 865,000 miles in diameter, from a direct photograph showing many sun-spots (Whitney) 21. Great sun-spot group, August 8, 1917 (Whitney) 22. Photograph of the hydrogen atmosphere of the sun (Ellerman) 23. Diagram showing outline of the 100-inch Hooker telescope, and path of the two pencils of light from a star when under observation with the 20-foot Michelson interferometer 24. Twenty-foot Michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the 100-inch Hooker telescope 25. The giant Betelgeuse (within the circle), familiar as the conspicuous red star in the right shoulder of Orion (Hubble) 26. Arcturus (within the white circle), known to the Arabs as the "Lance Bearer," and to the Chinese as the "Great Horn" or the "Palace of the Emperors" (Hubble) 27. The giant star Antares (within the white circle), notable for its red color in the constellation Scorpio, and named by the Greeks "A Rival of Mars" (Hubble) 28. Diameters of the Sun, Arcturus, Betelgeuse, and Antares compared with the orbit of Mars 29. Aldebaran, the "leader" (of the Pleiades), was also known to the Arabs as "The Eye of the Bull," "The Heart of the Bull," and "The Great Camel" (Hubble) 30.