180 A CENTURY'S PROGRESS IN ASTRONOMY. 



Hind and Pogson, were making remarkable dis- 

 coveries which greatly increased the number of 

 known variables. Among Hind's discoveries 

 were S Cancri of the Algol type ; while 

 Schmidt discovered another of the same class, 

 8 Librae, and also the famous Geminorum. 

 While director of the Observatory of Mannheim, 

 an institution equipped with very antiquated 

 instruments, Schonfeld devoted himself to the 

 study of variable stars, and increased the num- 

 ber of known variables considerably. In the 

 southern hemisphere Gould, in South America, 

 did for the observation of variable stars what 

 Argelander did in the northern. 



In 1874 a very important, although not so 

 obvious, service to variable-star astronomy was 

 rendered by the Danish observer, Hans Carl 

 Fredrik Christian Schjellerup (1827-1887). He 

 translated from Arabic into French the works 

 of the Persian astronomer of a thousand years 

 ago, Al-Sufi, and thus rendered his observations 

 available to modern astronomers. Al-Sufi was 

 a most accurate observer, and, by comparing 

 his catalogue with those of modern observers, 

 it can be found whether stars have changed 

 in brilliance during the past thousand years. 



The study of variable stars has been pursued 

 in recent years by many astronomers, both by 



