i 5 8 THE EVOLUTION OF 



of its decline, after which it shines steadily with its 

 original light for another period of fifty-nine hours, when 

 the variation is repeated, and so on indefinitely. The 

 position of Algol in Perseus, near the brilliant constella- 

 tion of Cassiopeiea, may be found from any star map, and 

 its variation in brightness can be readily followed by 

 comparing it from time to time with other stars in its 

 neighbourhood. 



The mathematical regularity of the variation led Good- 

 ricke in 1783 to suggest that it was caused by the 

 revolution of a dark companion round the star. The 

 orbit of the companion was assumed to lie in a plane 

 that made at most a very small angle with the line of 

 sight, so that at every revolution the star was partially 

 eclipsed. The theory obviously met the main facts of 

 the case, and it was very much strengthened by an 

 interesting investigation by E. C. Pickering in 1888. 

 From a great number of observations made by Schon- 

 field between 1862 and 1870, Pickering deduces the law 

 according to which the loss and the recovery of the star's 

 light was effected. Such a law is conveniently repre- 

 sented by a curve constructed upon the principle of the 

 ordinary barometer chart. Along a horizontal line equal 

 distances are set off to represent equal intervals of time, 

 and from each point verticals are drawn, each represent- 

 ing by its length the brightness of the star at the instant 

 indicated by the point from which the vertical is drawn. 

 A smooth curve is then drawn through the tops of the 

 verticals, and this is the light curve of the star. The 

 actual light curve of Algol is given in fig. 9. 



Having deduced the law of variation from Schonfield's 

 observations, Pickering proceeded to determine from 

 geometry the law according to which the light change 

 should take place, upon the assumption that it was caused 



