Miss Clerke says of Olbers, “Night after night, during half a century and upwards, he discovered, calculated, or observed the cometary visitants of northern skies.” He was the discoverer of the comet of 1815, known as Olbers’ comet. It moves round the Sun in a period of over seventy years, and returned to perihelion in 1887, forty-seven years after the death of its 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 repulsion, 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 luminous 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 in cometary astronomy by another great German astronomer, his friend and pupil Encke. The son of a Hamburg clergyman, Johann Franz Encke was born in that city in 1791, and died in 1865 at Spandau. After taking part in the war against Napoleon, he was in 1822 appointed director of the Gotha Observatory, being called to Berlin in 1825. In early life he was the pupil of Olbers and Gauss, and his investigations and discoveries formed an epoch in astronomy. His most famous discovery related to the little comet which bears his name. The comet was discovered by _J. L. Pons_ (1761-1831) at Marseilles, although it had previously been seen by Méchain and Caroline Herschel. In 1819 Encke computed the orbit of the comet, and boldly announced that it would reappear in 1822, its period being about 3¼ years, or 1208 days. In 1822 the comet, true to Encke’s prediction, returned to perihelion, and was observed at Paramatta in Australia, the perihelion passage taking place within three hours of the time predicted by Encke. As Miss Clerke remarks, “The importance of this event will be better understood when it is remembered that it was only the second instance of the recognised return of a comet; and that it, moreover, established the existence of a new class of celestial bodies, distinguished as comets of short period.”