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of Poland. He studied first at the University of Krakau, where astronomy was specially cultivated, and at the age of twenty-four he proceeded to Bologna, where he enjoyed the teaching of Domenico Maria Novara. Thus Copernicus not only became acquainted with Ptolemy's work, but also acquired some familiarity with the astrolabe or astronomical circle, one of the few crude instruments then in use. From about the end of 1505 till his death in 1543, Copernicus lived in the diocese of Ermland, in Prussia, most of the time in the town of Frauenburg, where he held a canonry at the cathedral. It is much to be regretted that we are utterly unacquainted with the manner in which Copernicus came to design the new system of astronomy which has made his name immortal. But he had probably early perceived that, however valuable the labours of Regiomontanus had been, they had not improved the theory of celestial motion, so that the most important problem, that of computing beforehand the positions of the planets and accounting for their apparently intricate movements, was practically untouched since the days of Ptolemy. That great mathematician had completed and extended the planetary system of Hipparchus, and had in a wonderfully ingenious manner represented the complicated phenomena. But more than 1400 years had elapsed since his time, and the system, however perfect from a mathematical point of view, had long been felt to be too complicated, and not agreeing closely enough with the observed movements of the planets. This circumstance led Copernicus to attempt the construction of a new system, founded on the idea that the sun, and not the earth, is the ruler of the planets. But though Copernicus on the basis of this idea developed a theory of the planetary movements as complete as that of Ptolemy, he was unable to do more than to demonstrate the possibility of explaining the phenomena by starting from