6 Mr. J. N. Lockyer. On the Classification [Apr. 12 r 



Fig. 1. — Suggested origin of the appearance presented by a planetary nebula. The* 

 luminosity is due to the collisions occurring along the sphere of 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 point outside. 



rent. The diagram shows the appearance on the supposition that the 

 conditions of all the orbits with reference to the major axis shall be 

 nearly identical, but the appearances would not be very greatly altered 

 if we take the more probable case in which there will be plus and 

 minus values. 



Globular Nebulce shoiving Condensation until finally a Nebulous Star 



is reached. 



If we grant the initial condition of the formation of a collision- 

 shell, we can not only explain the appearances put on by planetary 

 nebulae, but a continuation of the same line of thought readily explains 

 those various other classes to which Herschel has referred, in which 

 condensations are brought about, either by a gradual condensation 

 towards the centre, or by what may be termed successive jumps. 

 These condensations doubtless 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, 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 pericentric distance will at the 

 same time be reduced ; the swarm will, in consequence of this action, 



