Lassell on an Annular Nebula. 353 



LASSELL ON AN ANNULAR NEBULA. 



In a letter to M. Le Verrier, and by him communicated to 

 the French Academy, M. Lassell gives the following interesting 

 account :* — 



" On directing my great telescope to the planetary nebula 

 situated 20h. 56m., 101° 56', its structure appeared to me so 

 marvellous that I could not help sending you a drawing accom- 

 panied with a description. 



"By employing two magnifications of 231 and 285, I saw, 

 at first sight, an elliptical nebula of a clear blue, with a slight 

 prolongation, or rather a very faint star, towards the extremity 

 of its transverse axis. This aspect of the nebula resembled the 

 appearance of the planet Saturn, when its ring is seen in a nearly 

 full view. By employing still higher powers, magnifying re- 

 spectively 760, 1060, and 1480 times, and under the most 

 favourable circumstances, I discovered in the interior of the 

 nebula, a brilliant elliptical ring, perfectly sharp, and without 

 apparent connection with the surrounding nebula. This last is 

 like a thin veil of vapour, and not confounded with the margin 

 of the ring, whose splendour it diminishes very little. The 

 nebulous envelope, a little more removed from the extremity of 

 the conjugate axis than from the extremity of the transverse 

 axis, is in reality very fully prolonged, and it is difficult to 

 follow its traces amongst the stars that precede and follow it. 

 There is a star near its northern border in the prolongation of 

 its conjugate axis. The breadth or thickness of the ring differs 

 from that of Saturn in being nearly uniform throughout. It 

 appears, therefore, that if its form is really elliptical, we must 

 see in a direction almost perpendicular to its plane ; while if it 

 is actually circular, it is presented to us a little foreshortened. 

 A section passing through any portion of the space between 

 its inner and outer sides would be circular. In other words, it 

 is like a cylinder bent round till both ends meet. 



" At first I was inclined to refer it to the same class as the 

 annular nebula of Lyra, chiefly on account of its remarkable 

 central star, which was, however, of greater brilliance ; and, 

 besides this, the resemblance is incomplete, for the ring is much 

 more symmetrical, and better defined at its edges. It suggests 

 the idea of a compact assemblage of brilliant stars, like the 

 milky way. The brightness of the ring is not strictly uniform, 

 the south preceding position being slightly more luminous. 

 The transverse axis is inclined about 13 u to the parallel of de- 

 clination. A series of micrometrical measures of the length 

 and breadth of the ellipse gives a mean of 26"*2 for the trans- 

 * Comptes Rendus, October 13th, 1862. 



