1888.] 



of the various Species of Heavenly Bodies, 



89 



with double nuclei — incipient double stars, a smaller swarm revolving 

 round the larger condensation, or rather round their common centre 

 of gravity. In such a condition of things as this, it is obvious that, 

 as before stated, in the swarms having a mean condensation this 

 action is the more likely to take place, for the reason that at first the 

 meteorites are too sparse for many collisions to occur, and that, finally, 

 the outliers of the major swarm are drawn within the orbit of the 

 smaller one, so that it passes clear. The tables show that this view is 

 entirely consistent with the facts observed, for the greater number of 

 instances of variability occur in the case of those stars in which on 

 other grounds mean spacing seems probable. 



The Cases of Small Jiang e. 



So far, to account for the greatest difference in luminosity at 

 periastron passage, we have supposed the minor swarm to be only 

 involved in the larger one during a part of its revolution, but we can 

 easily conceive a condition of things in which its orbit is so nearly 

 circular that it is almost entirely involved in the larger swarm. 

 Under these conditions, collisions would occur in every part of the 

 orbit, and they would only be more numerous at periastron in the 

 more condensed central part of the swarm, and it is to this that I 

 ascribe the origin of the phenomena in those objects — a very small 

 number — in which the variation of light is very far below the normal 

 range, one or two magnitudes instead of six or seven. Of course, if 

 we imagine two subsidiary swarms, the kind of variability displayed 

 by such objects as ft Lyra? is easihy explained. 



Study of Lijht Curves* 



I owe to the kindness of Mr. Knott the opportunity of studying 

 several light curves of " stars " of this group, and they seem to entirely 

 justify the explanation which I have put forward. It is necessary, 

 however, that the curves should be somewhat carefully considered 

 because in* some cases the period of the minimum is extremely small, 

 as if the secondary body scarcely left the atmosphere of the primary 

 one but was always at work. But when we come to examine the 

 shape of the curves more carefully what we find is that the rise to 

 maximum is extremely rapid ; in the case of U Geminorum for instance 

 there is a rise of five magnitudes in a day and a half ; whereas the fall 

 to minimum is relatively slow. The possible explanation of this is 

 that the rise of the curve gives us the first sudden luminosity due to 

 the collisions of the swarms, while the descent indicates to us the 

 gradual toning down of the disturbance. If it be considered fair to 

 make the descending curve from the maximum exactly symmetrical 

 with the ascending one on the assumption that the immediate effect 

 produced is absolutely instantaneous, then we find in all cases that L 



