280 NOTE ON WILLIAM BALL'S OBSERVATIONS OF SATURN. [36 



It is well known that Huyghens's discovery of the true nature of the 

 appendage to Saturn, which had so puzzled Galileo and others, was con- 

 tested by Father Fabri at Rome, who wrote under the name of "Eustacius 

 de Divinis." 



Huyghens replied to Fabri's objections in a tract which appeared in 

 1660, entitled Brevis Assertio Systematic Saturnii sui, and which is contained 

 in the third volume of his collected works. 



In this tract he repeatedly appeals to Ball's observations in England 

 in confirmation of his own. It is clear that Huyghens was in possession 

 of drawings by Ball which represented the various appearances presented 

 by the planet during the four years from 1656 to 1659 inclusive, and that 

 he had carefully compared them with those which he had himself taken 

 during the same interval. After mentioning the dark band which he had 

 observed on the disk of Saturn at times when the remainder of the ring 

 was invisible, he quotes a letter from Dr Wallis, dated Dec. 22, 1658, in 

 which reference is made to an earlier letter dated May 29, 1656, wherein 

 Dr Wallis had mentioned this band as having been observed by Ball, and 

 had inquired whether his correspondent had likewise perceived it. Huyghens 

 goes on to say that from Feb. 5, 1656, to July 2, when the planet 

 appeared round and without ansae, this band or dark shading was observed 

 by Ball to cross the centre of the disk, as shewn in his drawing, exactly 

 as in Huyghens's own figure. 



Afterwards, when the ansse had re-appeared, the band was seen with 

 more difficulty, and its position was less accurately laid down in Ball's 

 drawing. From Nov. 5, 1656, to July 9, 1657, when the oblong arms of 

 Saturn were seen apparently united to the disk, Ball gives a figure quite 

 similar to that of Huyghens, except that he makes the arms a little thicker. 



Again, from Nov. 9, 1657, to June 7, 1658, when the arms were more 

 open, Ball's figure is exactly similar to Huyghens's, except a slight difference 

 in the position of the obscure zone or belt. 



Also, finally, the same remark applies to the figure of the planet from 

 Jan. 3, 1659, to June 17 of the same year, when the ansae were a little 

 more widely opened. 



Having made these comparisons between Ball's drawings of the planet 

 and his own, Huyghens remarks that Ball was unacquainted with his hypo- 

 thesis* (respecting the ring), and therefore could not be supposed to be 



* Huyghens's Systema Saturnium only appeared in 1659. 



