vigorous persecution of Protestants in his dominions, one step in which was an order that all Protestant ministers and teachers in Styria should quit the country at once (1598). Kepler accordingly fled to Hungary, but returned after a few weeks by special permission of the Archduke, given apparently on the advice of the Jesuit party, who had hopes of converting the astronomer. Kepler's hearers had, however, mostly been scattered by the persecution, it became difficult to ensure regular payment of his stipend, and the rising tide of Catholicism made his position increasingly insecure. Tycho's overtures were accordingly welcome, and in 1600 he paid a visit to him, as already described (chapter v., § 108), at Benatek and Prague. He returned to Gratz in the autumn, still uncertain whether to accept Tycho's offer or not, but being then definitely dismissed from his position at Gratz on account of his Protestant opinions, he returned finally to Prague at the end of the year.
138. Soon after Tycho's death Kepler was appointed his successor as mathematician to the Emperor Rudolph (1602), but at only half his predecessor's salary, and even this was paid with great irregularity, so that complaints as to arrears and constant pecuniary difficulties played an important part in his future life, as they had done during the later years at Gratz. Tycho's instruments never passed into his possession, but as he had little taste or skill for observing, the loss was probably not great; fortunately, after some difficulties with the heirs, he secured control of the greater part of Tycho's incomparable series of observations, the working up of which into an improved theory of the solar system was the main occupation of the next 25 years of his life. Before, however, he had achieved any substantial result in this direction, he published several minor works—for example, two pamphlets on a new star which appeared in 1604, and a treatise on the applications of optics to astronomy (published in 1604 with a title beginning Ad Vitellionem Paralipomena quibus Astronoiniae Pars Optica Traditur . . .), the most interesting and important part of which was a considerable improvement in the theory of astronomical refraction (chapter ii., § 46, and chapter v., § 110). A later optical treatise (the Dioptrice of 1611) contained a
