THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 321 



The most exact determinations of the period of rotation 

 were made by the diligent Scheiner (1630). Since the 

 strongest light which man has yet been able to produce, 

 Drummond's incandescent lime, appears of an inky black 

 when projected upon the sun's disk, we need not wonder that 

 Galileo, who doubtless first described the great solar faculae, 

 should have considered the light of the nuclei of the solar 

 spots to be more intense than that of the full moon, or of 

 the atmosphere near the solar disk. ( 492 ) Fancies respect- 

 ing the many envelopes of air, cloud, and light surrounding 

 the black earth-like nucleus of the sun, may be found in 

 the writings of Cardinal Nicolaus of Cuss, in the middle of 

 the 15th century. (^ 3 ) 



The cycle of admirable discoveries which scarcely occupied 

 two years, and in which the immortal name of the Florentine 

 shines foremost, was completed by the observation of the phases 

 of Yenus. As early as 1610 Galileo saw the sickle or crescent- 

 form of the planet, and, according to a practice already 

 alluded to, concealed the important discovery in an anagram, 

 which Kepler recals in the preface to his Dioptrica. He 

 says also, in a letter to Benedetto Castelli (Dec. 30, 1610), 

 that he thinks he has recognised changes in the enlightened 

 disk of Mars, notwithstanding the small power of his tele- 

 scope. The discovery of the moon-like crescent shape of 

 Yenus was the triumph of the Copernican system. The 

 necessity of the existence of these phases could certainly not 

 have escaped the founder of that system ; he discusses in 

 detail, in the tenth chapter of his first book, the doubts 

 which the later adherents of the Platonic opinions had 

 raised against the Ptolemaic system on account of the 

 moon's phases. But in the development of his own system 



