Solar prominences, photographed with the spectroheliograph without an eclipse (Ellerman) 31. The 150-foot tower telescope of the Mount Wilson Observatory 32. Pasadena Laboratory of the Mount Wilson Observatory 33. Sun-spot vortex in the upper hydrogen atmosphere (Benioff) 34. Splitting of spectrum lines by a magnetic field (Bacock) 35. Electric furnace in the Pasadena Laboratory of the Mount Wilson Observatory 36. Titanium oxide in red stars 37. Titanium oxide in sun-spots 38. The Cavendish experiment 39. The Trifid Nebula in Sagittarius (Ritchey) 40. Spiral nebula in Ursa Major (Ritchey) 41. Mount San Antonio as seen from Mount Wilson CHAPTER I THE NEW HEAVENS Go out under the open sky, on a clear and moon-less night, and try to count the stars. If your station lies well beyond the glare of cities, which is often strong enough to conceal all but the brighter objects, you will find the task a difficult one. Ranging through the six magnitudes of the Greek astronomers, from the brilliant Sirius to the faintest perceptible points of light, the stars are scattered in great profusion over the celestial vault. Their number seems limitless, yet actual count will show that the eye has been deceived. In a survey of the entire heavens, from pole to pole, it would not be possible to detect more than from six to seven thousand stars with the naked eye. From a single viewpoint, even with the keenest vision, only two or three thousand can be seen. So many of these are at the limit of visibility that Ptolemy's "Almagest," a catalogue of all the stars whose places were measured with the simple instruments of the Greek astronomers, contains only 1,022 stars. Back of Ptolemy, through the speculations of the Greek philosophers, the mysteries of the Egyptian sun-god, and the observations of the ancient Chaldeans, the rich and varied traditions of astronomy stretch far away into a shadowy past. All peoples, in the first stirrings of their intellectual youth, drawn by the nightly splendor of the skies and the ceaseless motions of the planets, have set up some system of the heavens, in which the sense of wonder and the desire for knowledge were no less concerned than the practical necessities of life. The measurement of time and the needs of navigation have always stimulated astronomical research, but the intellectual demand has been keen from the first. Hipparchus and the Greek astronomers of the Alexandrian school, shaking off the vagaries of magic and divination, placed astronomy on a scientific basis, though the reaction of the Middle Ages caused even such a great astronomer as Tycho Brahe himself to revert for a time to the practice of astrology. EARLY INSTRUMENTS