4 



Mr. J. N. Lockyer. On the Classification [Apr. 12, 



increase of optical power and improvement in observing conditions 

 and stations — may be held to strengthen the view that space is really 

 a meteoritic plenum, while the forms indicate motions and crossings 

 and interpenetrations of streams or sheets, the brighter portions 

 being due to a greater number of collisions per unit volume. 



From this point of view it is also possible that many stars, instead 

 of being true condensed swarms due to the nebulous development to 

 which we have referred, are simply appearances produced by the 

 intersection of streams of meteorites. They are, then, referable to 

 an intensification of the conditions which gave rise to the brighter 

 appearances recorded by Herschel here and there in his diffused 

 nebulosities. The nebulous appendages sometimes seen in connexion 

 with stars strengthen this view. 



When we come to the more regular forms we find that they may 

 be generalised into three groups, according as the formative action 

 seems working towards a centre ; round a centre in a plane or nearly 

 so ; or in one direction only. As a result we have globular, spheroidal, 

 and cometic nebulae. I propose to deal with each in turn. 



Globular Nebulrn. 



The remarkable appearance presented by the so-called planetary 

 nebulas requires that I should refer to them in some detail. Sir 

 William Herschel does not describe them at any great length, but in 

 his paper on " Nebulous Stars "he alludes to the planetary nebulosity 

 which in many cases is accompanied by a star in the centre, and 

 finally comes to the conclusion that " the nebulosity about the star is 

 not of a starry nature " (' Phil. Trans.,' vol. 81, 1791, p. 73.) 



Sir John Herschel, in his valuable memoir published in ' Phil. 

 Trans.,' 1883, describes them as "hollow shells" (p. 500). It was so 

 difficult to explain anything like their appearance by ordinary ideas 

 of stellar condensation that Arago, as quoted by Nichol (' Architec- 

 ture of the Heavens,' p. 86), abandoning altogether the idea that they 

 represented clusters of stars or partook in any wise of a stellar con- 

 stitution, imagined them as hollow spherical envelopes, in substance 

 cloudy and opaque, or rather semi-transparent ; a" brilliant body 

 invisible in the centre illuminating this spherical film, so that it was 

 made visible by virtue of light coming through it and scattered by 

 reflection from its atoms or molecules. 



Lord Rosse (' Phil. Trans.,' vol. 140, 1850, p. 507) records that 

 nearly all the planetary nebulae which he had observed up to that 

 time had been found to be perforated. In only one case was a perfo- 

 ration not detected, but in this ansae were observed, introducing into 

 the subject for the first time the idea of nebulous bodies resembling 

 to a certain extent the planet Saturn. But Lord Rosse, although he 

 thus disposed of the idea of Arago, still considered that the annular 



