The Planet Saturn. 195 



irregular refraction seems too extravagant to be admissible on 

 such, slender data. 



But if we pass over this difficulty as insoluble, we have yet 

 to encounter a still more obstinate one in accounting for the 

 abnormal character of the shadow of the ball projected upon 

 the ring. If the latter, considered as a whole, were an uniform 

 plane, the shadow on it would have an elliptic outline, deviating 

 more or less from parallelism with the edge of the globe, ac- 

 cording to the greater or less difference between the direction 

 of illumination and that of vision (an angle never exceeding a 

 small amount), but invariably curved in the same direction as 

 the limb, and perfectly smooth. That the ring is not an uniform 

 plane we have seen reason to believe, and we are so far pre- 

 pared to expect some slight deviations, but only such as would 

 be consistent with the extreme thinness, not only of the ring, 

 but even of its apparent protuberances, at the epoch of lateral 

 presentation. Now for the facts.* 



We have already seen that, as long ago as 1 792, Schroter 

 perceived an unevenness in the shadow, not only upon the ball, 

 but particularly, he says, upon the ring, where it was especially 

 distinguishable by a projecting portion of the light of A. The 

 next mention seems to be by Ij[, 1806, June 9, whose descrip- 

 tion of it does not, however, specify that feature of his diagram 

 which would now be thought most curious — the reversed cur- 

 vatures of the limb and its shadow. Then after a long- 

 interval, 1850, Lassell, Dawes, and Secchi saw the outline of 

 the shadow slightly concave towards the ball. Dawes thought 

 that it might be an illusion from contrast with the limb, or B 

 might be wedge-shaped, though then it would be hard to 

 account for its edgewise disappearance. Secchi referred it to 

 a slight convexity of B, experiment proving that a very slight 

 one would suffice ; or in some degree, possibly, to difference of 

 planes. — 1851, Sept. 5, Bond I. found the outline straight, or 

 a little concave. 9, Lassell ditto. 10, slightly concave. 25, 

 Dawes found the shadow on A irregular. — 1852, Oct. 5 to 

 middle, Lassell (Malta) saw a " remarkable sudden projection - " 

 of the shadow in crossing Ball's div., and an elongated spot on 

 B on the opposite (/) side, close to the limb, sometimes, but 

 not generally, extending to the division; this, he afterwards 

 perceived, was the germ of the coming shadow on that side, 

 as our eye travelled across the direction of illumination at the 

 opposition. 22, shadow on both sides ball : Bond II. 28, 

 ditto ; 2 wings on B also fancied : Bond II. Nov. 3, shadow 

 on both, sides; "what can this mean?'' C. W. Tuttle; also B. 



* Secchi alludes to Cassini and Messier as having inferred, from observation, 

 a curved surface for the rincr. I am n ot able to say whether this may signify any- 

 thing more than the irregularities of the edge-view. 



