1 66 



THE EVOLUTION OF 



of a long period of constant light, the variation is con- 

 tinuous, and instead of one minimum, there are two, but 

 one of them is more marked than the other. The com- 

 plete cycle of changes is effected in 12-9 days, and it may 

 be conveniently regarded as divided into four stages, 

 which are of almost equal duration, each lasting for rathei 



10 II 



i i 



15 



Days. 



FIG. 13. Light Curve of ft Lyrae. 



more than three days. Starting from the instant A, in 

 the figure, the time of the chief minimum, the brightness 

 of the star is 34 per cent, of its maximum. In the first 

 stage it rises rapidly to its maximum value of 100. It 

 then at once begins to decline, and in the second stage 

 falls to a secondary minimum with a brightness of 64. In 

 the third stage it rises again to 100, and in the last it 

 descends to its chief minimum of 34. The cycle is then 



