1888.] 



of the various Species of Heavenly Bodies, 



nebulae were really hollow shells, the perforation indicating an appa- 

 rently transparent centre. 



Hnggins and Miller subsequently suggested that the phenomena 

 represented by the planetary nebulae might be explained without 

 reference to the supposition of a shell (or a flat disk) if we consider 

 them to be masses of glowing gas, the whole mass of the gas being 

 incandescent, so that only a luminous surface would be visible ('Phil. 

 Trans,' vol. 154, 1864, p. 442). 



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 Dr. Huggins when his important discovery of the bright- 

 line spectrum of nebulae was given to the world. 



It remains, then, to see whether the meteoritic hypothesis can explain 

 these appearances when it is acknowledged that all the prior ones 

 have broken down. If we 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, the question arises whether there will not be at some 

 distance from the centre of the swarm a region in which collisions 

 will be most valid. If we can answer this question in the affirmative, 

 it will follow that some of the 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 peri- 

 .astron passage to a oonsiderable 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 ivill he 'practically 

 the only thing visible, and will present to us the exact and hitherto unex- 

 plained appearance of a planetary nebula — a body of the same intensity 

 of luminosity at its edge and centre — thus putting on an almost phos- 

 phorescent appearance. 



If the collision region has any great thickness, the centre should 

 appear dimmer than the portion nearer the edge. 



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. 1, 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 peri- 

 astron distance shall be somewhat considerable, the mechanism which 

 produces the appearance of a planetary nebula is at once made appa- 



