The time had now come for a more complete theory of comets than that of Olbers. The theory of electrical repulsion was developed in 1871 by Zöllner, 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. Zöllner 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 astronomer. Theodor Alexandrovitch Brédikhine, 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 representing the intensity of the repulsion fell into three classes. This was the first hint of a classification of cometary tails. Meanwhile he carefully studied the tails of comets both from direct observation and from drawings. In 1877 he wrote: “I suspect that comets are divisible into groups, for each of which the repulsive force is perhaps the same.” Subsequent investigations led Brédikhine to divide the tails of comets into three types. The first type consists of long, straight tails, pointed directly away from the Sun, represented by the tails of the great comets of 1811, 1843, and 1861. In the second type, represented by Donati’s and Coggia’s comets, the tails, although pointed away from the Sun, appear considerably curved. In the third type the tails are, to quote Miss Clerke, “short, strongly-bent, brushlike emanations, and in bright comets seem to be only found in combination with tails of the higher classes.”