586 



NATURh 



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tions of streams or sheets, the brighter portions being due 

 to a greater, number of collisions per unit volume. 



When we come to the more regular forrns we find that 

 they may be generalized 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 direc- 

 tion only ; as a result we have globular, spheroidal, and 

 cometic nebulcC. I propose to deal with each in turn. 



Globular NebiilcE. 

 The remarkable appearance presented by the so-called 

 planetary nebulae 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 

 ijiany 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. Ixxxi. 



P- 73, I79I)- 



Sir John Herschel, in his valuable memoir published in 

 Phil, Trans., 1833, 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 ("Architecture of the 

 Heavens," p. 86), abandoning altogether the idea that 

 they represented clusters of stars or partook in any wise 

 of a stellar constitution, imagined them as hollow spheri- 

 cal 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 re- 

 flection from its atoms or molecules. The mystery was 

 explained to a certain extent by Lord Rosse, who (Phil. 

 Trans., 1850, vol. cxl. p. 507) states that nearly all the 

 planetary nebute which he had observed with his colossal 

 instruments up to that time had been found to be per- 

 forated. In only one case was a perforation not detected, 

 but in this ans£e 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 nebulas were really 

 hollow shells, the perforation indicating an apparently 

 transparent centre. 



Huggins and Miller subsequently suggested that the 

 phenomena represented by the planetary nebulae might 



Fig. I.— Suggested origin of the appearance presented by a planetary nebula. The luminosity is due to the collisions occurring along the sphere cf 

 intersection of the elliptic orbits of the meteorites. The left-hand diagram is a cross-section of the meteoric system, and the right-hand one shows 

 the appearance of the collision shell as seen from a poin: outside. 



be explained without reference to the supposition of a 

 shell (or of a flat disk) if we consider them to be masses 

 of glowing gas, the whole mass of the gas being incand- 

 escent, so that only a luminous surface would be visible 

 (Phil. Trans., vol. cliv. p. 442, 1864). 



It will be seen that all these hypotheses are mutually 

 destructive ; but it is right that I should state, in referring 

 to the last one, that the demonstration that these bodies 

 are not masses of glowing gas merely has been rendered 

 possible by observations of spectra which were not 

 available to Messrs. Huggins and Miller when their im- 

 portant discovery of the bright-line spectrum of nebulae 

 was given to the world. 



It remains, then, to see whether the meteoritic hypo- 

 thesis can explain these appearances when it is acknow- 

 ledged that all the prior ones have broken down. 

 Let us for the sake of the greatest simplicity consider a 

 swarm of meteorites at rest, and then assume that others 

 from without approach it from all directions, their 

 previous paths being deflected. There will be at some 

 distance from the centre of the swarm a region in which 

 collisions will be most valid. Meteorites arrested here 

 •will begin to move in almost circular orbits round the 

 common centre of gravity. 



The major axes of these orbits may be assumed to be 

 not very diverse, and we may further assume that, to 

 begin with, one set will preponderate over the rest. Their 

 elliptic paths may throw the periastron passage to a con- 

 siderable distance from the common centre of gravity ; 

 and if we assume that the meteorites with this common 

 mean distance are moving in all planes, and that some 

 are direct and some retrograde, there will be a shell in 

 which more collisions will take place than elsewhere. 

 Now, this collision surface will be practically the only 

 thing visible, and will present to us the exact and 

 hitherto unexplained appearatice of a planetary fiebula — 

 a body of the same intensity of luminosity at its edge 

 and centre — thus putting on an almost phosphorescent 

 appearance. 



Such a collision surface, as I use the term, is presented 

 to us during a meteoric display by the upper part of our 

 atmosphere. 



I append a diagram, Fig. i, which shows how, if 

 we thus assume movement round a common centre of 

 gravity in a mass of meteorites, one of the conditions of 

 movement being that the periastron distance shall be 

 somewhat considerable, the mechanism which produces 

 the appearance of a planetary nebula is at once made 



