126 A CENTURY'S PROGRESS IN ASTRONOMY. 



discoverer. The great comet of 1811 was the 

 subject of a memoir which Olbers published the 

 following year, and in which he originated the 

 " electrical repulsion " theory of comets' tails. 

 Even after the fulfilment of Halley's great 

 prediction, comets were still looked upon with 

 profound awe, and the popular fear regarding 

 them was still prevalent. Olbers, however, 

 showed that the tails of comets resulted from 

 purely natural causes. He regarded the Sun 

 as possessed of a repulsive as well as an 

 attractive force, and considered the tails to 

 be vapours repelled from the nucleus of the 

 comet by the Sun. He calculated that in 

 the comet of 1811 the particles of matter 

 expelled from the head reached the tail in 

 eleven minutes, with a velocity comparable to 

 that of light. The theory of electrical repul- 

 sion, since elaborated by other observers, is 

 now generally accepted among astronomers. No 

 other hypothesis represents in such a complete 

 manner the formation and growth of the lum- 

 inous appendages of the celestial bodies so 

 picturesquely called "pale -winged messengers" 

 as that put forward by the physician of 

 Bremen. 



Some years after Olbers' famous theory was 

 given to the world, a great advance was made 



