326 cosMu.s. 



also, from a letter of his to Benedetto Castelli (30th of De- 

 cember, 1610), that he believed, notwithstanding the low mag- 

 nifying power of his telescope, that he could recognize changes 

 in the illumined disk of Mars, The discovery of the moon- 

 like or crescent shape of Venus was the triumph of the Coper 

 nican system. The founder of that system could scarcely fail 

 to recognize the necessity of the existence of these phases ; 

 and we find that he discusses circumstantially, in the tenth 

 chapter of his first book, the doubts which the more modern 

 adherents of the Platonic opinions advance against the Ptole- 

 maic system on account of these phases. But, in the develop- 

 ment of his own system, he does not speak expressly of the 

 phases of Venus, as is stated by Thomas Smith in his Optics. 

 The enlargement of cosmical knowledge, whose description 

 can not, unhappily, be wholly separated from unpleasant dis- 

 sensions regarding the right of priority to discoveries, excited, 

 like all that refers to physical astronomy, more general atten- 

 tion, from the fact that several great discoveries in the heavens 

 had aroused the attention of the public mass at the respective 

 periods of thirty-six, eight, and four years prior to the invention 

 of the telescope in 1608, viz., the sudden apparition and dis- 

 appearance of three new stars, one in Cassiopeia in 1572, an- 

 other in the constellation of the Swan in 1600, and the third 

 m the foot of Ophiuchus in 1604. All these stars were bright- 

 er than those of the first magnitude, and the one observed by 

 Kepler in the Swan continued to shine in the heavens for 

 twenty-one years, throughout the whole period of Galileo's dis- 

 coveries. Three centuries and a half have now nearly passed 

 since then, but no new star of the first or second magnitude 

 has appeared ; for the remarkable event witnessed by Sir 

 John Herschel in the southern hemisphere (in 1837)* was a 

 great increase in the intensity of the light of a long-known star 

 of the second magnitude (?/ Argo), which had not until then 

 been recognized as variable. The writings of Kepler, and our 

 own experience of the effect produced by the appearance of 

 comets visible to the naked eye, will teach us to understand 

 how powerfully the appearance of new stars, between the 

 years 1572 and 1604, must have arrested attention, increased 

 the general interest in astronomical discoveries, and excited 

 the minds of men to the combination of imaginative conject- 

 ures. Thus, too, terrestrial natural events, as earthquakes in 

 regions where they have been but seldom experienced ; the 

 eruption of volcanoes that had long remained inactive ; the 

 * Compare Cosmos, vol. i., p. 153 and 353. 



