of successive periods. The planets were the rulers of the elementary world and of the microcosmos, the moon being represented among the metals by silver, Mercury by quick- silver, Venus by silver, the sun by gold, Mars by iron, Jupiter by gold or tin, and Saturn by lead. It is therefore very probable that Tycho while working in the laboratory considered himself as merely for a while pursuing a special branch of the one great science, to the main branch of which he had hitherto felt specially attracted. But if these mystical speculations had as yet some power over his mind, they would seem gradually to have been pushed into the background, while cool and clear reasoning took their place, and guided him safely to his great goal—the reformation of practical astronomy.
We have now followed Tycho through what may be considered the first period of his life. By study and intercourse with learned men he had mastered the results of the science of antiquity and the Middle Ages. But though he had to some extent already, as a youth seventeen or eighteen years of age, perceived the necessity of a vast series of systematic observations on which to found a new science, he had hitherto shrunk from carrying out this serious undertaking himself, or had perhaps despaired of getting the means of doing so. But a most unusual and startling celestial phenomenon was now to occur, to rouse him to renewed exertion, and firmly fix in his mind the determination to carry out the plans he had so long entertained.
