The Evolution of the Sciences 



uniformly luminous and that their rotation 

 round an axis exposes to our view portions 

 of their disc possessing different lighting powers. 

 The existence of solar spots seems to corroborate 

 to some extent this explanation, which was 

 taken up again by Zollner. Maupertuis, with 

 greater daring, likened the periodic stars to 

 millstones, which sometimes show us their 

 edge and at others their flat side. There is 

 another hypothesis, maintained and developed 

 with great talent by Pickering, which considers 

 Algol as a binary system, consisting of a large 

 bright star and a smaller darker star, in the 

 rotation of which system the dark star partly 

 hides, " occults/' the bright star and causes 

 the decrease of brightness observed. 



If we apply the results of observation and 

 the law of universal gravitation to this hypothesis 

 we are led to the following conclusions: the 

 diameter of the dark companion of Algol would 

 be equal to that of the sun and the diameter 

 of the bright star would be about a fifth longer. 

 The two stars, situated about five million 

 kilometres apart, would turn round a centre 

 situated at about three-quarters of the distance 



248 



