6l6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1790. 



considerable suspicion, that by a refraction through some very rare atmosphere 

 on the 2 planes of the ring, the satellites might be lifted up and depressed, so 

 as to become visible on both sides of the ring, even though the ring should be 

 equal in thickness to the diameter of the smallest satellite, which may amount 

 to 1000 miles. As for the argument of its incredible thinness, which some 

 astronomers have brought from the short time of its being invisible, when the 

 earth passes through its plane, we cannot set much value on them; for they must 

 have supposed the edge of the ring, as they have also represented it in their 

 figures, to be square; but there is the greatest reason to suppose it either sphe- 

 rical or spheroidical, in which case evidently the ring cannot disappear for any 

 iDng time. Nay he ventures to say that the ring cannot possibly disappear on ac- 

 count of its thinness; since, either from the edge or the sides, even if it were 

 square on the corners, it must always expose to our sight some part which is il- 

 luminated by the rays of the sun : and that this is plainly the case, we may con- 

 clude from its being visible in the telescopes during the time when others of less 

 light had lost it, and when evidently we were turned towards the unenlightened 

 side, so that we must either see the rounding part of the enlightened edge, or 

 else the reflection of the light of Saturn on the side of the darkened ring, as we 

 see the reflected light of the earth on the dark part of the new moon. Dr. H. will 

 not however decide which of the 2 may be the case; especially as there are other 

 very strong reasons to induce us to think that the edge of the ring is of such a 

 nature as not to reflect much light. 



Dr. H. cannot leave this subject without mentioning both his own former sur- 

 mises, and those of several other astronomers, of a supposed roughness in the 

 surface of the ring, or inequality in the planes and inclinations of its flat sides. 

 They arose from seeing luminous parts on its extent, which were supposed to be 

 projecting points, like the moon's mountains; or from seeing one arm brighter 

 or longer than another; or even from seeing one arm when the other was in- 

 visible. He was, in the beginning of this season, inclined to the same opinion, 

 till one of these supposed luminous points quitted the edge of the ring, and ap- 

 peared to be a satellite. Now, as he had collected every inequality of this sort, 

 it was easy enough for him afterwards to calculate all such surmises by the known 

 periodical time of the several satellites; and he always found that such appear- 

 ances were owing to some of these satellites which were either before or behind 

 the ring. Oct. 20th, for instance, at 22^ SS*" 46% he saw 4 of Saturn's satel- 

 lites all in one row, and at almost an equal distance from each other, on the fol- 

 lowing side; and yet the 1st satellite, which was the farthest of them all, was 

 only about half-way towards its greatest elongation from the body of Saturn, as 

 may be seen in fig. 5. How easily, with an inferior telescope, this might have 

 been taken for one of the arms of Saturn, he leaves those to guess who know 



