DISCOVERIES IN THE CELESTIAL SPACES. 325 



but still I find nothing in his second remarkable letter of the 

 14th of August, 1612, to Marcus Welser, that would indicate 

 his having observed an inequality in the ash-colored margin 

 on both sides of the black nucleus when approaching the 

 sun's edge (Alexander Wilson's accurate observation in 1773). 

 The Canon Tarde in 1620, and Malapertus in 1633, ascribed 

 all obscurations of the sun to small cosmical bodies revolving 

 around it and intercepting its light, and named the Bourbon 

 and Austrian stars* [Borbonia et Austriaca Sidera). Fa- 

 bricius recognized, like Galileo, that the spots belonged to the 

 Bun itself;! he also noticed that the spots he had seen vanish 

 all reappear ; and the observation of these phenomena taught 

 him the rotation of the sun, which had already been conject- 

 ured by Kepler before the discovery of the solar spots. The 

 most accurate determinations of the period of rotation were, 

 however, made in 1630, by the diligent Scheiner. Since the 

 strongest light ever produced by man, Drummond's incan- 

 descent lime-ball, appears inky black when thrown on the 

 sun's disk, we can not wonder that Galileo, who undoubtedly 

 first described the great solar faculce, should have regarded 

 the light of the nucleus of the sun's spots as more intense than 

 that of the full moon, or the atmosphere near the sun's disk.J 

 Fanciful conjectures regarding the many envelopes of air, 

 clouds, and light, which surround the black, earth-like nucleus 

 of the sun, may be found in the writings of Cardinal Nicholas 

 of Cusa as early as the middle of the fifteenth century, s^ 



To close our consideration of the cycle of remarkable dis- 

 coveries, which scarcely comprised two years, and in which the 

 great and undying name of the Florentine shines pre-eminent, 

 it still remains for us to notice the observation of the phases 

 of Venus. In February, 1610, Galileo observed the crescentic 

 form of this planet, and on the 11th of December, 1610, in 

 accordance with a practice already alluded to, he concealed 

 this important discovery in an anagram, of which Kepler 

 makes mention in the preface to his Dioptrica. We learn 



* Delambre, Hist, de V Astronomie Moderne, t. i., p. 690. 



t The same opinion is expressed in Galileo's Letters to Prince Cesi 

 (May 25, 1612) ; Veuturi, Part i., p. 172. 



X See some ingenious and interesting considerations on this subject 

 by Arago, in the Annuaire pour Van 1842, p. 481-488. Sir John Her- 

 schel, in his Astronomy, $ 334, speaks of the experiments with Drum- 

 mond's light projected on the sun's disk. 



$ Giordano Bruno und Nic. von Cusa verglichen, von J. Clemens, 

 1847, s. 101. On the phases of Venus, see Galilei, OpereA. ii., p. 53, 

 and Nelli, Vita, vol. i., p. 21.3-215. 



