Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: Fio. 1. —The Great 3tiuch Telescope of the Lick Observatory. tions of double stars make it practically certain that the law of gravitation holds true in every part of the visible universe. The double stars are the ouly ones, except the sun, whosewith respect to each other. In the year 134 B.C. a bright star, visible even in the daytime, suddenly appeared where none previously had been known, and this stimulated the astronomer Hipparchus (about 180-110 B.c.), to make a catalogue of all the stars he could easily see, 1080 in number. Since the invention of the telescope the possibilities in this line have been enormously increased. The instrument which has been almost universally used is a small telescope delicately mounted so as to turn in the plane of the meridian. The observations are excessively laborious and entirely devoid of anything bordering on the sensational. The most extensive piece of work carried out by a single observer is Argelander's catalogue of 324,198 stars in the northern hemisphere. About fifteen years ago a plan was developed by international cooperation for obtaining a more exhaustive catalogue of the whole sky by photographic processes. The theory of making and reducing observations was revolutionized by the great German astronomer Bessel (1784— 1846), who is known as the father of modern practical astronomy. (2) The study of the moon, sun, and planets has been carried on with large telescopes, which are mounted so that they may be readily pointed to any part of the sky. By their aid nearly all that is known about the moon and the planets has been discovered. In the case of the sun the spectroscope has been of relatively greater value. (3) In the search for comets a comparatively small telescope, with low magnifying power, mounted so that it ...