1 82 A Short History of Astronomy [Cn. vn 



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 ha'l 

 hopes of converting the astronomer. Kepler's hearers had, 

 however, mostly been scattered by the persecution, it be- 

 came difficult to ensure regular payment of his stipend, 

 and the rising tide of Catholicism made his position in- 

 creasingly 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 w..s 

 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 pos- 

 session, but as he had little taste or skill for observing, the 

 loss was probably not great ; fortunately, after some diffi- 

 culties 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 ex- 

 ample, 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 Astronomiae Pars Optica Traditur . . .), 

 the most interesting and important part of which was a 

 considerable improvement in the theory of astronomical 

 refraction (chapter 11., 46, and chapter v., no). A 

 later optical treatise (the Dioptrice of 1611) contained a 



