OP ARTS AND SCIENCES. 193 



indicates his failure to see the jagged appearances of the principal 

 division ; and indeed, he could not have expected to see it, as these 

 forms are almost as difDcult to make out as the grayish line of the 

 outer ring. 



I have no positive evidence that these markings continued visible 

 after the end of September, 1875 ; as, after that time, I discontinued 

 somewhat ray observations on Saturn, looking at it only occasionally, 

 until the present year, during which, I have observed it on every pos- 

 sible occasion. But, of course, the obliquity of the Ring is too great 

 now to allow the observation of such delicate forms, although I still 

 continue to see the principal division on the ansae. It is not impossible 

 that the obliquity of the Ring was the cause of the failure of Professor 

 Hall to verify my observations, or the phenomenon may be a temporary 

 one, and it may have been absent when he made his observations. 



The phenomenon of the jagged border of the principal division, as 

 I have represented it, was seen so often and with such distinctness in 

 1872, when the Ring was wide open, that it was impossible for me to 

 doubt its reality ; and, besides, it was verified at least on two occasions 

 by Professor Winlock, the late director of the Harvard Observatory, 

 who once was accompanied by Mr. Milikeu, manager of the Western 

 Union Telegraph Company, who also saw the dark angular forms on 

 the following ansa. 



Professor Hall seems to be in doubt as to the reality of the anoma- 

 lous curvature of the shadow of the planet on the Ring, and appears 

 inclined to attribute this appearance to soine illusion caused by the varied 

 conditions of our atmosphere. In reply, I will remark that, if such 

 was the case, how could we explain its long duration as concave, and 

 its no shorter duration as couvex, which has -been alternatively observed 

 since the time of Cassini? 



Several years ago, a very distinguished and industrious observer, 

 F. Angelo Secchi of the Roman Observatory, pointed out that the 

 deformation of the shadow of the Ball on the Ring was the natural con- 

 sequence of the unevenness of the surface receiving it. If this is the true 

 explanation, as I think it is, the natural consequence, as derived from 

 the observations, is that the form of the surface is not permanent, 

 since the shadow has evidently shown different outlines ; appearing at 

 different times either as a straight, a convex, a concave, or a notched 

 line. 



Cambridge, Oct. 5, 1877. 

 VOL. xiii. (n.s. v.) 13 



