Oqtober 2, 1890] 



NATURE 



549 



The smaller the range, the more will both maxima and 

 minima be affected by the subsidiary swarms. W Cygni is 

 a case in point. •' 



It has been before remarked jthat the hypothesis de- 

 mands that in sparse swarms .of n)eteorites (Groups I. 

 and II.) the ascent to maximum, due to the sudden action 

 of the colliding swarms, should be much more rapid than 

 the descent to minimum, for the reason that the descent 

 must represent z. gradual cooXxxv^ down of the disturbance. 

 This more rapid ascent has been noted in 



R Piscium 



S Vulpeculse 



R Leonis Minoris 



R Ursas Majoris 



R Corvi 



W Cygni 



S Cassiopeiae ... 



R Arietis 



R Orionis 



T Delphini 

 T Vulpeculse 



Known Group II. stars. 



Group not yet determined. 



I have also suggested that the short minimum is a 

 measure of the indirect disturbance, but it is easy to 



imagine that this short minimum will not be invariable 

 under all conditions, and accordingly we find in R Persei 

 with a period of 212 days, a long rriinimum. 



In stars of Group VI., on the other hand, where we 

 have simply to deal with th^ added light of comets 

 passing perihelion, the<e is no reason why this should 

 happen, indeed, it ought to be rather the other way, 

 since comets put on their greatest brilliancy after peri- 

 helion. As a matter of fact, so far as my inquiries have 

 gone, I have not yet come across a case of a Group VI. 

 star showing any great difference in the times spent in 

 rising and falling. 



On the hypothesis a perfectly constant period can only 

 occur in the case of those double swarms in which the 

 central one has a regular figure and density. The 

 moment this condition is departed from, seeing that the 

 central swarm is certain to be in rotation, variation ot 

 period as well as of maximum must be expected. 



Nor is this the only variation which depends upon the • 

 central body. In the absence of knowledge in each case, 

 we must assume that the structure of the central swarm 

 resembles that of those which have been examined in 

 Andromeda, Ursa Major, and Canes Venatici — that is, 

 the meteoritic density will vary locally (S Aquilae), and 



Fig. 6.— Light-curve of U Cygni, showing Mr. Knott's actual observations. 



some of the observations made may be explained on the 

 supposition that the subsidiary swarm breaks into regions 

 in which the density is suddenly increased, as if we were 

 noting the result of a ring being pierced (R Aurigae). 



We have only to look at Mr. Roberts's photograph of 

 the nebula in Andromeda, and consider under what 

 different conditions a secondary swarm might reach the 

 same periastric distance if there were any rotation in the 

 nebula or any movement of the nodes, to recognize the 

 importance of taking the above points into consideration. 



If there be a condition of the central body anything like 

 that of the nebulas named, it must be borne in mind that 

 in the struggle for existence those swarms moving in the 

 plane of the intakes and in the same direction, will be 

 those that will longest survive ; hence we ought to be able 

 to explain the light-curves on the supposition that the 

 conditions of the secondary swarms are as stated above, 

 and it is seen that we can so explain them. 



When we have more than one subsidiary swarm it is 

 easy to see that certain relations of the regular periods of 

 their orbital motions will produce an irregularity in the 

 compound period ; so that a rhythmic change of period 



NO. 1092, VOL. 42] 



will enable us to learn somewhat of the relation of the 

 relative intensity and period of each of the swarms. We 

 are really in presence of a light-tide, the elements of 

 which can be found by analysis, as occurs with other 

 tides nearer home. 



The explanation suggested by the hypothesis of the 

 variability of stars of Group VI. seems also to throw light 

 upon the strange colours of some of them. R Leporis, 

 for instance, one of the most marked variables in the group, 

 is the famous crimson star observed by Mr. Hind. Now- 

 crimson = red + blue. All these stars are red, and in 

 many of them the absence of the blue is one of the most 

 emphasized characteristics of the spectrum. ; 



But suppose that the secondary swarm which adds its 

 light at maximum is a comet with the usual carbon bands, 

 we shall get this condition of things : — 



Blue. Green. Citron. 



! masked by \ 

 continuous > absorbed ... absorbed 

 absorption ) 

 Bands in subsidiary swarm ... bright ... bright ... bright 



