588 



NATURE 



{April 19, 1888 



nebulae at all. I am aware that in Newton's opinion the 

 cause of this rotation was not mechanical, but the 

 moment we assume a meteoric origin of these globular 

 clusters it is straining the facts to assume that the in- 

 take will be exactly the same at all points, and the 

 moment the bombardment is more or less localized, 

 rotation must follow sooner or later. Sir William 

 Herschel, in his paper of 181 1 (p. 319), says: "If 

 we consider this matter in a general light, it appears 

 that every figure which is not already globular must 

 have eccentric nebulous matter, which, in its en- 

 deavour to come to the centre, will either dislodge 

 some nebulosity which is already deposited, or slide 

 upon it sideways, and in both cases produce a circular 

 motion ; so that, in fact, we can hardly suppose a possible 

 production of a globular form without a subsequent 

 revolution of nebulous matter, which in the end may 

 settle in a regular rotation about some fixed axis." 



Given, then, a globular swarm with a rotation around 

 an axis, we have to discuss the phenomena produced by 

 collisions under a new set of circumstances. 



Here at once we have to account for the fact that the 

 nearly spherical forms are very short-lived, for they are 

 very rare ; we seem to jump, as it were, from globes to 

 very extended spheroids. 



If it be conceded that from the above considerations 

 we are justified in supposing that the elliptic and other 

 spheroidal nebulae really represent a higher stage of evo- 

 lution than those presented to us in the globular form, it 

 is clear that on the meteoritic hypothesis the greater part 

 of the phenomena will represent to us what happens to 

 such a system under the condition of a continuous 

 bombardment of meteorites from without. 



So soon as we have a minor axis, there will at firet be 

 more collisions parallel to it ; the result of this will be that 

 the equatorial plane will be intensified, and then, later on, 

 if we conceive the system as a very extended spheroid, 

 it is obvious that meteorites approaching it in direc- 

 tions parallel to its minor axis will now have fewer 

 chances of collisions than those which approach it, from 

 whatever azimuth, in what we may term the equatorial 

 plane. These evidently, at all events if they enter the 

 system in any quantity, will do for the equatorial plane 

 exactly what their fellows were supposed to do for the 

 section in Fig. i, and we shall have on the general 

 background of the symmetrically rotating nebula, which 

 may almost be invisible in consequence of its constituent 

 meteorites all travelling the same way and with nearly 

 equal velocities, curves indicating the regions along which 

 the entrance of the new swarm is interfering with the 

 movements of the old one ; if they enter in excess from 

 any direction, we shall have broken rings or spirals. 



This was suggested in my last paper. Various rings 

 will indicate the regions where most collisions are possible, 

 and the absence of luminosity in the centre by no means 

 demonstrates the absence of meteorites there. 



Researches by Lord Rosse and others have given us 

 forms of nebulae which may be termed sigmoid and 

 Saturnine, and these suggest that they and the elliptical 

 nebulas themselves are really produced by the rotation 

 of what was at first a globular rotating swarm of meteor- 

 ites, and that in these later revelations we pick up those 

 forms which are produced by the continued flattening of 

 the sphere into a spheroid under the meteoric con- 

 ditions stated. It is worthy of remark that all the 

 forms taken on by the so-called elliptic nebulae described 

 by the two Herschels, and by the spiral, sigmoid, and 

 Saturnine forms which have been added to them by the 

 labours of Lord Rosse and others, are recalled in the 

 most . striking manner by the ball of oil in Plateau's 

 experiment, when rotations of different velocities are 

 imparted to it. It is my intention to repeat Plateau's 

 experiments, and to take instantaneous photographs of 

 the various phenomena presented, and to place them side 



by side with the drawings of nebulae, of which they are 

 almost the exact counterparts. 



The Saturnine form may, indeed, in some cases re- 

 present either the first or last stages in this period of the 

 evolutionary process. I say may represent, in conse- 

 quence of the extreme difficulty in making the observa- 

 tions, so that in the early stages a spherical nebula, 

 beginning to change into a spheroid, may have its real 

 spheroidal figure cloaked by various conditions of 

 illumination. 



The true Saturnine form must, as in the case of Saturn 

 itself, represent one of the latest forms in the meteor- 

 swarm, because, if it be not continually fed from without, 

 collisions must sooner or later bring all the members of 

 the swarm to the centre of figure. 



Coinetic Nebula. 



I do not know that any explanation has, so far, been 

 suggested as to the origin of these curious forms, which 

 were first figured by Sir William Herschel, and of which 

 a number have recently been observed in the southern 

 hemisphere (" Melbourne Observations"). It is clear 

 that in them the conditions are widely different from 

 those hitherto considered in this paper. I think that the 

 meteoritic hypothesis satisfactorily explains them, on the 

 supposition that we have either a very condensed swarm 

 moving at a very high velocity through a sheet of meteor- 

 ites at rest, or the swarm at rest surrounded by a sheet 

 all moving in the same direction. It is a question of 

 relative velocity. 



If we consider the former case, it is clear that the 

 collision region will be in the rear of the swarm, that the 

 collision will be due to the convergence of the members 

 of the sheet due to the gravity of the swarm, and that 

 the collision region will spread out hke a fan behind the 

 swarm. 



The angle of the fan, and the distance to which the 

 collisions are valid, will depend upon the velocity of the 

 condensed swarm. 



Nebulous Origin of some Bodies which appear as Stars. 



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, simply produced 

 by an intensification of the conditions which gave rise to 

 the brighter appearances recorded by Herschel here and 

 there in his diffused nebulosities. The nebulous append- 

 ages sometimes seen in connection with stars strengthen 

 this view. 



II. Stars with Bright Lines or Flutings, 



I pointed out in my last paper that those stars in the 

 spectra of which bright lines had been observed were in 

 all probability the first result of nebulous condensation, 

 both their continuous spectrum and that of the surround- 

 ing vapour being produced by a slightly higher tempera- 

 ture than that observed in nebula in which similar 

 though not identical phenomena are observed. 



I have recently continued my inquiries on this point ; 

 and I may say that all I have recently learned has 

 confirmed the conclusions I drew in my last paper, 

 while many of the difficulties have disappeared. Be- 

 fore I refer to these inquiries, however, it is necessary to 

 clear the ground by referring to the old view regarding 

 the origin of bright lines in stellar spectra, and to the 

 question of hydrogen. 



Reference to the Old View by which it was supposed 

 some of the Bright-line Phenomena might be accounted 

 for. 

 In the views which, some years ago, were advanced by 



myself and others, to account for the bright lines seen 



