October 2, 1890] 



NATURE 



547 



In the hypothetical case represented in Fig. I the 

 constant light of the central swarm may be taken as 6 mag., 

 and the added light of the two secondary swarms as vary- 

 ing from nil to 3 mag. and from nil to 4^ mag. respectively. 

 It is then obvious that the integrated effects of the light 



added produce constant maxima of I4'85 units, and minima 

 alternately o and 2 '97. We can in this way represent the 

 light-curve of a star which changes its magnitude from 

 3 to 4^ and 3 to 6 alternately. 

 The relative scales of light-units to brightnesses shown 



Fig. 3.— Hypothelkal curve in light-units. 



Fig, 4.— Hypothetical curve in magnitudes. 



by the foregoing figures, however, enable us to transpose 

 the diagram to one in which equal spaces represent 

 equal differences of magnitudes. This is shown in 

 Fig. 2. 



In the diagrams, the light-curves of the two subsidiary 

 swarms are represented by dotted lines, and the integrated 



result by the continuous line. One of the revolving 

 swarms has a period of 6 units of time, and the other a 

 period twice as long. The eccentricity of the primary 

 swarm is such that it adds, at maximum, 1485 light-units, 

 while the secondary swarm adds 2*97 light-units, 

 A comparison of the two diagrams will make clear what 



