xii TOWARDS INFINITY 321 



brightness is diminished by about two- thirds, and three, 

 and a half hours later it has regained the former intensity, 

 which continues unaltered for another fifty-nine hours. 

 These variations succeed one another with clock-like 

 regularity, so that the times when the brightness of 

 Algol will fade can be tabulated years in advance, as 

 they are in the Nautical Almanack and similar publica- 

 tions. A navigator or other traveller with such a table 

 at hand can correct his watch by observing when the 

 star dims in brightness, and comparing the tabulated 

 time of this occurrence with the time shown by his 

 chronometer. 



To explain the sudden reduction in light of Algol, 

 Goodricke suggested that a dark body is revolving 

 around the star, and periodically comes between us and 

 it, thus causing a partial eclipse in each revolution. 

 As a star can never be seen as anything but a point of 

 light, whatever telescopic means are employed, it is 

 impossible to distinguish any outlines of a dark body 

 upon a luminous disc, such as is seen, for instance, 

 during a partial eclipse of the sun by the moon, but 

 Goodricke's explanation was accepted, because it 

 accounted for the observed variations. The proof of the 

 hypothesis was not forthcoming for more than a century 

 after Goodricke suggested it ; and then it was reached 

 by indirect methods. Assuming that Algol has a dark 

 companion, the two bodies must swing round their 

 common balancing point, or centre of gravity. When the 

 dark body is moving towards us before passing in front 

 of the bright globe, the latter must be swinging back ; 

 and when the dark companion is receding after the 

 eclipse, the bright star must be approaching. Algol 

 must, therefore, alternately recede and approach in a 

 period which coincides with that of its changes of 



