“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 considerable 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 appearance of a planetary nebula—a body of the same intensity of luminosity at its edge and centre—thus putting on an almost phosphorescent 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.”[13] [Footnote 13: _Proc. of Royal Society_, vol. xliv. p. 5.] _Spheroidal nebulæ_, he considers, are produced by the rotation of what was at first a globular rotating swarm of meteorites. _Cometic nebulæ_ are explained, he considers, “on the supposition that we have either a very condensed swarm moving at a very high velocity through a sheet of meteorites at rest, or the swarm at rest surrounded by a sheet, all moving in the same direction.” In an able and interesting work, which seems almost utterly unknown in England,[14] Professor Winchell has advanced views similar to those of Tait and Lockyer regarding the nature and origin of nebulæ. But he, in addition, discusses the further question of the origin of those swarms. I shall have occasion to refer to Professor Winchell’s views more fully when we come to the consideration of the pre-nebular condition of the universe. [Footnote 14: _World Life, or Comparative Geology_, by Alexander Winchell, LL.D., Professor of Geology and Palæontology in the University of Michigan. Chicago: S. C. Griggs & Co. 1883.]