178 A CENTURY'S PROGRESS IN ASTRONOMY. 



the nineteenth century the number of known 

 variable stars was very small, as a glance at the 

 list given in Brewster's edition of Ferguson's 

 Astronomy (1811) will show. Some remarkable 

 investigations were due to the English astron- 

 omer, John Goodricke (1764-1786), who redis- 

 covered the variability of the star Algol, and 

 accurately determined its period in 1782. Good- 

 ricke suggested that the regular variations in 

 the light of Algol were due to the partial 

 eclipse of its light by a dark satellite, a hypo- 

 thesis now fully confirmed. Two years later, in 

 1784, Goodricke discovered other two variables, 

 8 Cephei and y8 Lyreo. He died in 1786 at the 

 age of twenty-one, and thus variable-star astron- 

 omy was deprived of its founder. 



The foundation of variable-star astronomy as 

 an exact branch of the science is due to Arge- 

 lander. In the years 1837-1845, while residing 

 at Bonn during the erection of the observatory, 

 of which he had been made director, he erected 

 a temporary observatory, and there he carefully 

 determined the magnitudes of all stars visible in 

 Central Europe. From this he was led to the 

 discussion of stellar variation, to which subject 

 he continued to give much attention. He was 

 the first to describe a method of observing 

 variable stars scientifically and accurately, a 



