90 THE HISTORY OF SCIENCE 



tion being made by means of one of the sights, the 

 parallel horizontal slits in which were aligned with 

 the corresponding parts of the circumference of the 

 cylinder. The altitude was recorded according to 

 the position of the sight attached to the graduated 

 arc. 



Tycho Brahe had a great reverence for Copernicus, 

 but he did not accept his planetary system ; and he 

 felt that advance in astronomy depended on pains- 

 taking observation. For over twenty years under the 

 kings of Denmark he had good opportunities for 

 pursuing his investigation. The island of Hven be- 

 came his property. A thoroughly equipped observa- 

 tory was provided, including printing-press and 

 workshops for the construction of apparatus. As 

 already implied, capable assistants were at the as- 

 tronomer's command. In 1598, after having left 

 Denmark, Tycho in a splendid illustrated book ( As- 

 tronomioe Instauratce Mechanica) gave an account of 

 this astronomical paradise on the Insula Venusia as 

 he at times called it. The book, prepared for the 

 hands of princes, contains about twenty full-page 

 colored illustrations of astronomical instruments (in- 

 cluding, of course, the mural quadrant), of the ex- 

 terior of the observatory of Uraniborg, etc. The 

 author had a consciousness of his own worth, and 

 deserves the name Tycho the Magnificent. The re- 

 sults that he obtained were not unworthy of the 

 apparatus employed in his observations, and before 

 he died at Prague in 1601, Tycho Brahe had con- 

 signed to the worthiest hands the painstaking record 

 of his labors. 



Johann Kepler (1571-1630) had been called, as 



