April 19, 1888] 



NATURE 



$S7 



apparent. The diagram shows the appearance on the sup- 

 position that the conditions of all the orbits with reference 

 to the major axis shall be nearly identical, but the appear- 

 ances would not be very greatly altered if we take the more 

 probable case in which there will be plus and minus values. 



Globular 'Nebulce showing Condensations until finally 

 a Nebulous Star is reached. 

 If we grant the initial condition of t'le formation of 

 a'coUision-shell, we can not only explain the appearances 



put on by plnnetary nebulae, but a continuation of the 

 same line of thouglit readily explains those various other 

 classes to which Herschel has referred, in which con- 

 densations are brought about, either by a gradual con- 

 densation towards the centre, or by what may be termed 

 successive jumps, showing that they are among the earliest 

 stages of nebular development. 



To explain these forms we have only to consider what 

 will happen to the meteorites which undergo collision in 

 the first shell. They will necessarily start in new orbits, 



Fig. 2.— Suggestion as to the ongm of a globular nebula w itb a bnghcer central portion. A.s in the former case, the luminosity of the fainter portion is dne 

 to the c jUis'ons which occur along the sphere of intersection represented by the la-ger circle. Aftir collision the meteorites will travel m new orbits, 

 \i and there will be an additional sphere of intei^sec on, rep evented by ihe smaller circle. Tne left-han 1 diagram is a cross-section, and the right-hand 

 one represents the appearance of the two collision shells as seen from a point outside. 



Fig. 3. — Suggestion as to the origin of a nebulous star. The orbits of the inner .set of meteorites are very elliptic, so that the shell of intersection appears 

 almost as a point. As in the previous ca-sei, the left-hand diagram represents the miteoric systems in section, and the right-hand one the appearance 

 from a point outside. 



and it is suggested that an interior collision-shell will in 

 this way be formed, 



In consequence of the collisions the orbits will have a 

 tendency to get more and more elliptic, while the peri- 

 centric distance will at the same time be reduced ; the 

 swarm will, in cor, sequence of this action, gradually 

 brighten towards the centre through collisions being 

 possible nearer the centre, and ultimately we shall have 

 nebulae with a distinct nucleus^ the nucleus then repre- 



senting the locus of most collisions. This brightness 

 may be sudden in places, or quite gradual, according to 

 the collision conditions in each swarm. The final stage 

 will be a nebulous star. 



Effects of Subsequent Rotation. — Spheroidal Nebulce. 



In such meteor-swarms as those we have considered, 

 it must be that rotation is sooner or late.- s^t up. Other- 

 wise it would be impossible to account for the spheroidal 



