NEBULAE. 319 



criterion by which to prove the existence of a mutual relation 

 between the two, as in distinguishing between physically, and 

 merely optically double stars. Figures of double nebulae are 

 given in the Philos. Transact, for the year 1833, figs. 68-71- 

 Compare also Herschel, Outlines of Astr. 878; Observ. at 

 the Cape of Good Hope, 120. 



Annular nebula? are of the rarest occurrence. According 

 to Lord Rosse, we are acquainted with seven of these bodies 

 in the northern hemisphere; the most celebrated of these is 

 situated between ft and 7 Lyra (No. 57, Messier; No. 3023 

 of Sir John Herschel's Catalogue), and was discovered in 

 1779 by Darquier at Toulouse, when Bode's Comet passed 

 near it. Its apparent size is nearly equal to that of Jupiter's 

 disc, and its form is an ellipse, whose greater and lesser axes 

 are in the ratio of 5 to 4. The interior of the ring is not 

 black, but somewhat illumined. Sir William Herschel disco- 

 vered some stars in the ring, which has since been entirely 

 resolved by Lord Rosse and Mr. Bond. 47 The splendid 

 annular nebula? of the southern hemisphere, numbered 3680 

 and 3686, appear, on the contrary, perfectly black in the 

 interior of the rings. The last-named of the two is not ellipti- 

 cal, but perfectly round; 43 all are probably annular clusters of 

 stars. The increasing power of optical instruments appears, 

 moreover, generally to render the contour of both elliptical 

 and annular nebula) less defined; thus, for instance, Lord 



47 "Annular Nebula:" Observations at the Cape, p. 53; 

 Outlines of Astr. p. 602. " Nebuleuse perforee :" Arago, 

 in the A nnuaire pour 1842, p. 423; Bond, in Schum. Astron. 

 Nachr. No. 611. 



48 Observations at the Cape, p. 114, pi. vi. figs. 3 and 4. 

 Compare also No. 2072 in the Philos. Transact, for 1833, 

 p. 466. See Nichol, Thoughts on the System of the World, 

 p. 21 7 pi. iv. and p. 22, pi. i. fig. 5. 



