The light-curve of a variable star 557 
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 part 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 way. 
Examples of light curves will be found in Figs. 5 and 7 below. 
Jan. 1, 1900 Scale of hours 
2 121 
Mag = 7a99—j_ 7+? _4 6 6 7 6 9 19 19 19 14 125 16 14, 
ba 28 2-2, leas 
‘sof on “ 4 +30 
ras c e 
aa} f° % f \ | 42 
“45 + fe ‘ a a5 
>} f ‘ Ad Ly 
= ra x / \ “8 
L y ; ‘ a 
Boa yj . } e 61 
® 4 ‘i ® ¢ \ be 
Ss 67 ¢ \ i \ a 
Bl cok ! \ F ‘ 60 
oy i ‘ : ® 
Oo 63 f a * ‘ 3 
3 66 ei ‘ / \ 63 
oS -69 i i / iy 68 
a ate ; \ Pi a 13 
wt \ , \ f \ 76 
78 A y, ‘eb ‘e 4138 
81 ‘ef Swe" 21 
784 ‘i i : i ; : ; A wid 
Fig. 5. 
Light curve of RR 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 ove of the stars which 
furnishes the most striking results in connection with the object 
of this essay’. This star has a mean magnitude of about 74, and it is 
therefore invisible to the naked eye. Its period of variability is 
142 32™ 10°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 spaced 
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 
1 See Monthly Notices R.A.S. Vol. 63, 1903, p. 527. 
