420 



Mr. J. N. Lockyer. On the Cause of [Nov. 21, 



Only revolving swarms with large orbits will therefore be effective 

 in producing variability, but even these will only cause variability of 

 short range, since the number of collisions at periastron passage will 

 be small, the swarm being sparse. In the sparsest swarms, therefore, 

 the variability will be of a long period and the range will be small. 

 These are no doubt the causes of the variability having been over- 

 looked. 



When we pass to the mean swarms, however, the variability 

 becomes more strongly marked. Cometic swarms of short period, if 

 they exist at all, will still only produce a general brightening of the 

 central swarm, and the swarms most effective in producing variability 

 will therefore be those with moderately long periods. The range of 

 variability will depend upon the eccentricity of orbit and the periastron 

 distance of the revolving swarm, as in the general case. 



As the central swarm becomes more and more condensed, and 

 therefore gradually loses its redness, only shorter period swarms will 

 be effective in producing variability, as the outliers will have been 

 drawn entirely within the orbits of longer period swarms, if they 

 exist at all. 



Still further condensation of the central swarm will draw the 

 outliers within the orbits of the revolving swarms, which would 

 produce variability in the swarms last considered, and now only very 

 short period swarms are concerned. At the same time the colour 

 wall have become yellow or yellowish- white, the swarm having 

 passed from Group II to Group III. 



It will be seen that my theory perfectly explains the general 

 relation of period to colour which has been observed by Chandler and 

 previously by Schmidt,* and in fact demands it. 



The range of variability does not appear to bear any relation to the 

 periodicity (except perhaps in the sparsest swarms), and this is only 

 what we should expect, as the conditions on which the range depends 

 (periastron distance, and eccentricity of orbit of revolving swarm) 

 are special to each star. Cometic swarms in our own system follow 

 no general rule as regards the eccentricities of their orbits, or their 

 perihelion distances. 



IV. The Irregular Variables of Group II. 



The next test is that of irregularity. The apparent irregularities 

 in the variability of stars in the group under discussion are, on my 

 theory, produced by the revolution of several swarms of meteorites 

 at different rates and different distances round the central one. The 

 swarms most subject to irregularity should, therefore, on this view, 

 be those which are most likely to suffer from the effects of the 



* Quoted in ' Observatory,' Feb., 1889. 



