between the north pole and two degrees of south declination. Leaving the execution of these observations to his assistants, Argelander started with Struve, in June, 1852, by way of Göttingen and Berlin, to Sweden and Finland, and thence to Pultova, where he spent four weeks, to return at the end of September with the best impressions and in full vigor. The winter and spring of 1854 were so favorable to observations, while the reduction-tables were not far behind them, that Argelander could rejoice in the thought that the atlas would contain from fifty to a hundred thousand more stars than he had originally contemplated. Ever careful to preserve the unity of the whole, he arranged most of the materials himself, and was not willing to let a zone pass without personally working upon it. Double stars and stars with greater proper motions were noted; Winnecke's and Kruger's determinations of parallax were connected with these, and the restoration of lost stars from the older observations afforded ever-new interest. The revisions went on till the summer of 1861; but in some of the regions they no longer required all of Argelander's time, and he was able to pursue other objects. He identified all the stars of the eighth magnitude and brighter, which were not found in the earlier catalogues, with the accessible variable stars, the stars for comparison with the Mannheim observations of nebulæ and for former appearances of periodical comets, and especially many stars with presumed or determined proper motions. Next, while still occupied with the collation of his material, Argelander turned his eye to a work he had long contemplated, of more exact meridian observations of all the stars to the ninth magnitude, of which the Durchmusterung had made known the general positions. The labor, too much for a single establishment, was to be divided among different observatories. So, in the summer of 1865, he made a proposition for the simultaneous observation of selected stars at different points, by which he hoped to obtain material for the investigation of star catalogues in general, and to secure the needed number of fixed points for the greater work. The plan for this work was first presented, in 1867, to the officers of the University at Bonn, and afterward at the general meeting of the Astronomische Gesellschaft, where the author also gave his views respecting the most convenient way of carrying it out. With some slight modifications they form the basis of the programme decided upon by the society at Vienna in 1869.
Argelander undertook a small part of the preliminary work for the execution of this scheme, but found that, at the age of seventy years, he was no longer competent to make the necessary observations, and he gave them over to his assistants, carrying on himself only the minor series. The main part of the treatise on the subject was printed only a short time before his death.
