184 A CENTURY'S PROGRESS IN ASTRONOMY. 



suffering partial eclipse only because the plane 

 of the orbit lies in our line of sight. Algol, 

 therefore, is not inherently a variable star, but 

 merely a binary. Following up his researches, 

 Vogel, assuming that the bright and dark stars 

 are of equal density, arrived at the conclusion 

 that Algol is a globe about one and a half 

 million miles in diameter, the satellite equalling 

 the size of the Sun, and the centres of the stars 

 being separated by about 3,230,000 miles. Thus, 

 star- variables of the Algol type are not variable 

 in the true sense of the word. Even the most 

 irregular of the Algol variables have been ex- 

 plained. Perhaps the most irregular was Y 

 Cygni, discovered by Chandler in 1886. It was 

 soon found, however, that the variations recurred 

 with great irregularity : in less than two years 

 the phases differed by as much as seven hours 

 from the predicted times. At length the subject 

 was taken up by Dune'r at Upsala. A series of 

 observations made with the 14-inch refractor at 

 Upsala in 1891 and 1892 convinced him in the 

 latter year that two eclipses take place in the 

 course of one revolution : one star occults the 

 other. Duner showed that the intervals between 

 minima were thus 1 day 9 hours; 1 day 15 

 hours; 1 day 9 hours, and so on. Thus, the 

 first, third, fifth, and seventh sets of minima 



