36] NOTE ON WILLIAM BALL'S OBSERVATIONS OP SATURN. 281 



biased by it, while he himself would not dare to represent the phenomena 

 otherwise than they really were, since, if he did, he might at once be 

 contradicted by the English observer. 



This judgment of so competent an authority as Huyghens, made while 

 he had before him all the materials for forming it, left no doubt on my 

 mind as to the merit of Ball's observations. 



In order to see whether any further light could be thrown on the 

 subject, I have recently taken an opportunity of consulting the MSS. pre- 

 served in the archives of the Royal Society. 



Among them I find there is a letter in William Ball's own hand, 

 dated April 14, 1666, in which he makes reference to his observations of 

 Saturn, although the greater part of the letter relates to other subjects. 

 He mentions that the observations were made partly with a telescope thirty- 

 eight feet in length, having a double eye-glass, and partly with another 

 telescope twelve feet in length. In the postscript to this letter he gives 

 a small sketch of Saturn as it appeared at that time (1666), and he men- 

 tions that the same appearance was presented by the planet in 1664. In 

 this figure the external boundary of the ring has the form of a regular 

 oval, without any notches or other irregularities. 



No allusion is made to the very different appeai*ance which, if the 

 figure in the Philosophical Transactions is authentic, the planet must have 

 presented in 1665. 



It should be understood that the paper in the Philosophical Transactions 

 which is now in question was not written by Ball himself. It contains, 

 however, a quotation from a letter of Ball to a friend (probably Sir R. 

 Moray), and in what appears to be the last clause of this quotation, the 

 figure is said to be "a little hollow above and below." I cannot help 

 thinking that this clause has been added or altered in some way to correspond 

 with the given figure. The letter of Ball on which this paper was founded 

 is not in the archives ; but there is preserved, not a drawing, but a paper- 

 cutting, representing the planet and its ring, which is no doubt the original 

 of the figure engraved in the Transactions. 



The defect in the paper-cutting probably originated in the following 

 way. In order to make the cutting, the paper was first folded twice in 

 directions at right angles to each other, so that only a quadrant of the 

 ellipse had to be cut. 



The cut started rightly in a direction perpendicular to the major axis, 

 but through want of care, when the cut reached the minor axis, its direction 



A. 36 



