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



formed a slightly obtuse angle with that axis instead of being perpendicular 

 to it. 



Consequently, when the paper was unfolded, shallow notches or depres- 

 sions appeared at the extremities of the minor axis. 



I imagine that the account in the Philosophical Transactions was written 

 by some one inexperienced in astronomical observations, who took for granted 

 that the figure was correct. The mistake being soon discovered, the plate 

 which contained the erroneous figure of Saturn, together with two other 

 figures relating to different subjects, was cancelled, and thus its appearance 

 in only a few of the copies is accounted for. The other figures on the 

 cancelled plate were repeated in a new plate which accompanied No. 24 

 in the same volume of the Transactions. 



In Lowthorp's abridged edition of the Transactions the figure of Saturn 

 has been corrected. 



I find no evidence that Ball, any more than Huyghens, had noticed 

 any indication of a division in the ring. 



It may be interesting to give the original text of the passages of 

 Huyghens's Brevis Assertio Systematis Saturnii sui, in which reference is 

 made to Ball's observations. 



The citations are taken from the third volume of Huyghens's Opera 

 Varia, edited by 'S Gravesande, and published at Leyden in 1724. 



"Credo et fasciam nigricantem in Saturni disco, liquido sibi conspici 

 dixisset Eustacius, ni Fabrio visum fuisset earn nimium hypothesi meae 

 annular! favere. Cum autem ne optimis quidem suis perspicillis earn cerni 

 affirmet, hinc quoque quanto ilia meis deteriora sint perspicuurn sit. Nam 

 ne mihi phenomenon illud confictum credatur, idem et in Anglia pridem 

 observari ccepisse sciendum est ; et liquet ex literis viri clar. Joh. Wallisii, 

 Oxonia ad me datis 22 Dec. 1658, quibus inter alia hsec scribit. Monebam 

 etiam iisdem literis (nempe datis 29 Maji 1656) de Saturni fascia quam jam 

 ante observaverat D. Ball, et sciscitabar num tu eandem conspexeras, &c. 

 Earn porro fasciam a 5 Feb. 1^56 ad 2 Jul., quo tempore rotundus Saturnus 

 absque ansis apparuit, medium planetae discum secare D. Ball adnotavit, ut 

 in schemate ad me misso expressa est. Atque ita mihi quoque fuerat eo 

 tempore observata, ut cernitur pag. 544 Systematis Saturnii, quam figuram 

 hie repeto. Postmodum tamen renatis Saturni ansis cum difficillime conspici 

 eadem fascia ccepisset, minus recte quoque a D. Ball, quantum ad situm 

 attinet, depicta est. At in mearum observationum adversariis, die 26 Nov. 



