Astronomy and Photography at Rome. 381 



" 1841, May 3, 6h. 40m. of the evening. — The summit of the 

 lucid border of the spot towards the dark part of Venus was no 

 longer seen with the magnifying power of 240. With the other 

 of 824 it was hardly seen. With less magnifying powers the 

 dark part of the disc appeared dentellated at that point. 



" At 6h. and 50m. the lucid edge of the planet was not plainly 

 terminated by a circular curve ; towards the southern horn it ap- 

 peared deficient ; from thence by little and little it turned to be 

 visibly terminated in a circular curve, but between it and the 

 limit of the dark part there was visible a very subtile but well 

 decided and long black spot, like a slender thread stretched from 

 one horn towards the other." 



" 1841, May 4, at the same hour. — The darkness had covered 

 entirely the half of the elliptic edge of the crater ; and the north- 

 ern peak of it appeared terminated in a triple point. Two of 

 those points were the effect produced by the edge of the darkness 

 upon the brink of the erater, which was seen to penetrate with 

 two lucid arms into the dark part. Magnifying power, 824, and 

 afterwards 1128." 



The excellence of the telescope used by Fontana is particularly 

 noticed by Gassendi. It was this, with his own adroitness and 

 acute vision, which enabled Fontana to be the lucky discoverer 

 of the spots on the discs of Venus and Mars, and of the spots and 

 belts on that of Jupiter ; but it was reserved for that truly great 

 man, Dominic Cassini, to pursue the inquiry with success, about 

 twenty years afterwards ; to assign with precision the situation 

 of the spots, and, by calculation, to determine the periods of the 

 rotation on their axis, for each of those planets, with wonderful 

 accuracy. His observations and calculations for Mars and Jupi- 

 ter, were at once generally admitted ; but, although he was, in 

 reality, more successful in the instance of the rotation of Venus 

 than in that of either of the other two, for he estimated it at 23 

 hours, 21 minutes, which was only 22 seconds less than the truth, 

 yet, by a strange perversity, this was especially doubted, and his 

 accuracy, and almost his veracity, called in question. He had 

 seen the spots of Venus at Rome, but in vain tried to discover 

 them with the telescope of the observatory at Paris. He persist- 

 ed in his assertions and in his calculations ; and the northern as- 

 tronomers persisted in their doubts, to his great discomfort, and 

 that of his worthy son, J. J. Cassini, who like JEneas, with be- 



