132 A CENTURY'S PROGRESS IN ASTRONOMY. 



theory of comets than that of Olbers. The 

 theory of electrical repulsion was developed in 

 1871 by Zollner, whose principle of investigation 

 is thus described by Miss Clerke : " The efficacy 

 of solar electrical repulsion relatively to solar 

 attraction grows as the size of the particle 

 diminishes." If the particle is small enough, 

 it will obey the repulsive, and not the attractive, 

 power of the Sun. Zollner considered that the 

 smallest particles of comets obeyed the repulsive 

 power, and thus formed the tails of comets. The 

 development of a complete cometary theory is 

 due, however, to the genius of a Russian astron- 

 omer. Theodor Alexandrovitch Bredikhine, 

 born in 1831 at NicolaiefF, was employed at 

 Moscow Observatory from 1857 to 1890, when 

 he was promoted to the position of director at 

 Pulkowa. He resigned in 1895, and spent his 

 last years in St Petersburg, where he died on 

 May 14, 1904. From the beginning of his 

 astronomical career he was devoted to the 

 study of comets and their tails, but it was the 

 appearance of Coggia's comet in 1874 which 

 marked the commencement of his most important 

 observations. In that year, on making certain 

 calculations regarding the hypothetical repulsive 

 force exerted by the Sun on various comets, he 

 reached the conclusion that the values repre- 



