32 COSMOS. 



prove the existence of a mutual relation between the two, as 

 in distinguishing loetween physicallij 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,h 120. 



Annular nebulae aire 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 /3 and y Lyrae (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 

 disk, 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 dis- 

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

 resolved by Lord Rosse and Mr. Bond.^ The splendid an- 

 nular nebulae 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 elliptical, but 

 perfectly round ;t all are probably annular clusters of stars. 

 The increasing power of optical instruments appears, more- 

 over, generally to render the contour of both elliptical and 

 annular nebulae less defined ; thus, for instance, Lord E-osse's 

 colossal telescope exhibits the annular nebula of Lyra in the 

 form of a simple elHpse, with remarkable divergent, thread- 

 like nebulous appendages. The transformation effected in a 

 nebulous spot — Lord Rosse's Crab nebula — which appears in 

 instruments of inferior power to be a simple elliptical body, 

 is particularly striking. 



The so-called planetary nebulae, which were first observed 

 by the elder Herschel, and which rank among the most re- 

 markable phenomena of the heavens, although of less rare 

 occurrence than annular nebulae, do not number, according 

 to Sir John Herschel, more than 25, of which nearly three 

 fourths lie within the southern hemisphere. These bodies 

 present the most striking resemblance to planetary disks ; the 



* " Annular Nebulce.''^ — Ohservations at the Cape, p. 53 ; Outlines of 

 Astr., p. 602. " Nebulcuse perfor^e.'^ — Arago, in the Annuaire for 1842, 

 p. 423; Bond, in Schum., Asiron. Nachr., No. 611. 



t Ohservations at the Cape, p. 114, pi. vi., figs. 3 and 4. Compare 

 also No. 2072 in the Philos. Transact, for 1833, j). 466. See Nichol, 

 Thoughts on the System of the World, p. 21, pi. iv., and p. 22, pi. L 

 fig 5. 



