100 A CENTURY'S PROGRESS IN ASTRONOMY. 



being found, sometimes as many as twenty being 

 discovered in a single year. Some astronomers 

 have made the search for asteroids their chief 

 business. The principal asteroid discoverers have 

 been Christian H. F. Peters (1813-1890), Henri 

 Perrotin, Paul Henry (1848-1905), Prosper 

 Henry (1849-1903), James Watson, Robert 

 Luther (1822-1900), Johann Palisa (born 1848), 

 and Max Wolf (born 1863). 



In 1891 a new impulse was given to asteroid 

 study by the application of photography by Max 

 Wolf to the discovery of the minor planets. It 

 occurred to Wolf that the asteroid would be 

 represented on the plate by a trail, caused by 

 its motion during the time of exposure ; and 

 assisted by Arnold Schwussmann (born 1870), 

 Luigi Camera (born 1875), and others, Wolf has 

 discovered over a hundred asteroids, and he 

 has the whole field of asteroid hunting to him- 

 self. Few minor planets are now discovered by 

 the older method. In 1901 Wolf invented his 

 new instrument of research, the stereo -com- 

 parator, which, on the principle of the old- 

 fashioned stereoscope, represents the planetary 

 bodies as suspended in space far in front of the 

 stars. In this way this ingenious astronomer 

 has been enabled to discover asteroids at the 



