Scanned by Charles Keller with OmniPage Professional OCR NOTE: degrees A (Absolute?) is the same as the current degrees K (Kelvin). THE POPULAR SCIENCE MONTHLY VOLUME LXXXVI JULY TO SEPTEMBER, 1915 THE SCIENTIFIC MONTHLY VOLUME I OCTOBER TO DECEMBER, 1915 EDITED BY J. McKEEN CATTELL THE SCIENTIFIC MONTHLY ------ OCTOBER, 1915 ------------- THE EVOLUTION OF THE STARS AND THE FORMATION OF THE EARTH. II BY DR. WILLIAM WALLACE CAMPBELL DIRECTOR OF THE LICK OBSERVATORY, UNIVERSITY OF CALIFORNIA THE PRINCIPLES OF SPECTROSCOPY THUS far our description of the stellar universe has been confined to its geometrical properties. A serious study of the evolution of the stars must seek to determine, first of all, what the stars really are, what their chemical constitutions and physical conditions are; and how they are related to each other as to their physical properties. The application of the spectroscope has advanced our knowledge of the subject by leaps and bounds. This wonderful instrument, assisted by the photographic plate, enables every visible celestial body to write its own record of the conditions existing in itself, within limits set principally by the brightness of the body. Such records physicists have succeeded to some extent in duplicating in their laboratories; and the known conditions under which the laboratory experiments have been conducted are the Rosetta Stones which are enabling us to interpret, with more or less success, the records written by the stars.