The organised search for minor planets was relinquished in 1816, presumably because no more planets seemed to be forthcoming, and it does not appear that any further attempts were made by anybody till about 1830, when a Prussian amateur, named Hencke of Driessen, profiting by the publication of some new star maps put forth by the Berlin Academy, commenced a methodical search for small planets. These Berlin maps, one for each hour of R. A., were only completed in 1859, and, therefore, Hencke had only a small number of them at his command during the early years of his labours. Still it is strange that 15 years elapsed before his zeal and perseverance were rewarded, his first discovery, the planet Astræa, not taking place till December 1845. Once however the ice was broken new planets followed with considerable rapidity, and beginning with 1847, no year has elapsed without several or many having been found. During the last decade the number detected annually has been very great—sometimes as many as 20 in a year, but this has been the result of photography being brought to bear on the work. It is obvious that if a photograph of a given field taken on any one day is compared with a photograph taken a few days earlier or later, and any of the objects photographed have moved, their change of place will soon be noticed and will be a distinct proof of their planetary nature. It seems quite certain that all the larger of these planets have now been found, for the average brilliancy (and this no doubt means the average size) of those recently discovered has been steadily diminishing year by year, and it looks as if the limit of visibility will soon be reached, if it has not been reached already. The three largest of these bodies, in order of size, have generally been thought to be Vesta, Ceres, and Pallas; but Barnard, from observations made in 1894, concluded that Ceres is 520 miles in diameter; Pallas, 304 miles; and Vesta, 241 miles. As to all the rest of the minor planets, excepting Juno, Hornstein is of opinion that those having a greater diameter than 25 geographical miles are few in number, and that the majority of them are no larger than from 5 to 15 miles in diameter.