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304 Kepler therefore began to consider whether the theory might not after all be wrong, though it represented the longitudes so well; but during the short time he was at Benatky he was unable to make any progress in this problem, which it eventually took him four years to solve.

During Kepler's residence with Tycho at Prague between October 1600 and April 1601, he seems to have been mainly occupied with a piece of work which cannot have been congenial to him—a refutation of the book of Reymers Bär. Shortly after his arrival in Bohemia, Tycho began to take legal proceedings for libel against this person, who had fled to Silesia, from whence he, however, secretly came back some time afterwards. In the summer of 1600 Tycho learned that there was periculum in mora, as Reymers was very ill; but even this did not soften Tycho's heart, and he persisted in having the poor wretch punished, and persuaded the Emperor to appoint a commission of four members, two barons and two Doctors of Law, to try the libeller. But just as the trial was about to commence, Reymers died, on the 15th August 1600. The Emperor directed the Archbishop of Prague to have every obtainable copy of the book confiscated and burned, while Tycho, who was rather unduly proud of his system of the world, wished to publish a book which was to contain all the documents on the subject of the alleged plagiarism, as well as a scientific refutation of Reymers' book. The preparation of the latter had to be undertaken by Kepler, who, while battling with intermittent fever in 1601, wrote his Apologia Tychonis contra Ursum, in which he showed that neither Apollonius of Perga nor