150 HISTORY OF THE [1860-70 to treat their linear movements by reference to rectangular co-ordi- nates with the sun as origin and the pole on the axis of Z. He illustrated the application of his method by treating the proper motions of 113 stars, taken from Main's recent determinations, and deduced an apex of the sun's motion on each of two suppositions ; first, that the irregularities of proper motions are entirely due to chance error of observations ; and second, that such irregularities are entirely due to the peculiar motions of the stars. A couple of years later, Dunkin had applied Airy's method to the discussion of the proper motions of 1167 stars from Main's values ; and the results gave positions of the apex, both for Airy's 113 stars and for Dunkin's 1167 stars as follows : Airy, 113 Stars. Dunkin, 1167 Stars. ist Supposition R.A. 256 D. +39-5 261 4- 39-5 2nd Supposition 261 +247 263 +25-o To compare with Herschel 260 +2o,'3 Argelander 256 +38-6 Airy recognised, of course, that his method, as then applicable, had to proceed on the incorrect assumption that the stars were all at the same distance from the sun ; and he expected to find that light would be thrown upon the matter by the way in which the sum of the squares of the residual errors was affected. But no definite conclusion was forthcoming, " I therefore asked Mr. Stone to examine the matter, as I may say, maliciously, to discover if there were not some error : he has gone into it and can find nothing wrong. Supposing Bradley made errors of right ascen- sion, that might account for a good deal ; but the matter is left in a most delightful state of uncertainty, and I shall be very glad if anyone can help us out of it." Much water has flowed under the bridges, as the saying is, since then. There has come an immense development of statistical methods in the last twenty years. But the episode is peculiarly characteristic of Airy's philosophical mind. To the year 1863 belongs also the beginning of another centre of co-operation, the birth of the Astronomische Gesellschaft, which was formed at Heidelberg on August 28, with a Council containing the names of Argelander, O. Struve, Bruhns, Schonfeld, Zollner, Zech, and Foerster. With the death of the last-named, in 1921 January, the last personal link with the original founders is gone. De la Rue, in addressing the Society after the recess in the first year of his Presidency, 1864, gave a very interesting account of a visit paid by him to Pulkowa, in compliance with an invitation which he had received as President of our Society, to take part in
