320 COSMOS. 



Rosse's colossal telescope exhibits the annular nebula of Lyra 

 in the form of a simple ellipse, with remarkable divergent, 

 thread-like nebulous appendages. The transformation effected 

 in a nebulous spot Lord Rosse's Crab nebula which ap- 

 pears 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 amongst the most 

 remarkable 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 discs; the 

 greater number are round, or somewhat oval, and either 

 sharply defined or indistinct and vaporous at the margins. 

 The discs of many of these nebulae present a very uniform 

 light, whilst others appear mottled, or of a peculiar texture 

 as if curdled. No trace of condensation round a central point 

 has ever been observed. Lord Rosse has recognized five 

 planetary nebulous spots to be annular nebulae, having one or 

 two central stars. The largest of these planetary nebulae is 

 situated in the Great Bear (near /3 Ursae Maj.) and was dis- 

 covered by Mechain in 1781. The diameter of the disc 49 is 

 2' 40". The planetary nebula in the Southern Cross (No 

 3365, Observations at the Cape, p. 100), with a disc having a 

 diameter scarcely equal to 12", exhibits the brightness of a 

 star of the 6*7th magnitude. Its light is indigo-blue, and the 



49 If we consider the planetary nebula in the Great Bear to 

 be a sphere having an apparent diameter of 2' 40", "and 

 assume its distance to be equal to the known distance of 61 

 ^ygni, we shall obtain an actual diameter for the sphere, 

 which is seven times greater than the orbit described by 

 Neptune." Outlines, 876. 



