The light-curve of a variable star 567 



3 and 4, and then again perhaps it may return to between 2 and 3, 

 and so forth. With practice he learns to evaluate the brightness down 

 to small fractions of a magnitude, even a hundredth pari of a 

 magnitude is not quite negligible. 



For example, in observing the star RR Centauri five stars were in 

 general used for comparison by Dr Roberts, and in course of three 

 months he secured thereby 300 complete observations. When the 

 period of the cycle had been ascertained exactly, these 300 values 

 were reduced to mean values which appertained to certain mean 

 places in the cycle, and a mean light-curve was obtained in this tray. 

 Examples of light curves will be found in Figs. 5 and 7 below. 



Fig. 5. 

 Light curve of ER Centauri. 



I shall now follow out the results of the observation of RR 

 Centauri not only because it affords the easiest way of explaining 

 these investigations, but also because it is one of the stars which 

 furnishes the most striking results in connection with the object 

 of this essay 1 . This star has a mean magnitude of about 7L and it is 

 therefore invisible to the naked eye. Its period of variability is 

 14 h 32 m 10 8 76, the last refinement of precision being of course only 

 attained in the final stages of reduction. Twenty nine mean values of 

 the magnitude were determined, and they were nearly equally Bpaced 

 over the whole cycle of changes. The black dots in Fig. 5 exhibit the 

 mean values determined by Dr Roberts. The last three dots on the 

 extreme right are merely the same as the first three on the extreme 

 left, and are repeated to show how the next cycle would begin. The 



i See Monthly Noticeo Ii.A.S. Vol. 63, 1903, p. 527. 



