From the beginning of the observations the declination circle is fixed at a certain declination (whole degrees). All stars passing the field at a distance smaller than one degree from the middle line are observed. Hence the name “Durchmusterung”. When a star passes the “hour line” the magnitude is called out by A, and noted by B together with the time of the clock. Simultaneously the declination is noted by A in the darkness. On some occasions 30 stars may be observed in a minute. The first observation was made on Febr. 25, 1852, the last on March 27, 1859. In all there were 625 observation nights with 1841 “zones”. The total number of stars was 324198. The catalogue was published by ARGELANDER in three parts in the years 1859, 1861 and 1862[7] and embraces all stars between the pole and 2° south of the equator brighter than 9m.5, according to the scale of ARGELANDER (his aim was to register all stars up to the 9th magnitude). To this scale we return later. The catalogue is arranged in accordance with the declination-degrees, and for each degree according to the right ascension. Quotations from B. D. have the form B. D. 23°.174, which signifies: Zone +23°, star No. 174. ARGELANDER's work was continued for stars between δ = -2° and δ = -23° by SCHÖNFELD, according to much the same plan, but with a larger instrument (aperture 159 mm, focal length 1930 mm, magnifying power 26 times). The observations were made in the years 1876 to 1881 and include 133659 stars.[8] The positions in B. D. are given in tenths of a second in right ascension and in tenths of a minute in declination. 20. _The Cape Photographic Durchmusterung_[9] (C. P. D.). This embraces the whole southern sky from -18° to the south pole. Planned by GILL, the photographs were taken at the Cape Observatory with a DALLMEYER lens with 15 cm. aperture and a focal-length of 135 cm. Plates of 30 × 30 cm. give the coordinates for a surface of 5 × 5 square degrees. The photographs were taken in the years 1885 to 1890. The measurements of the plates were made by KAPTEYN in Groningen with a “parallactic” measuring-apparatus specially constructed for this purpose, which permits of the direct obtaining of the right ascension and the declination of the stars. The measurements were made in the years 1886 to 1898. The catalogue was published in three parts in the years 1896 to 1900. The positions have the same accuracy as in B. D. The whole number of stars is 454875. KAPTEYN considers the catalogue complete to at least the magnitude 9m.2. In the two great catalogues B. D. and C. P. D. we have all stars registered down to the magnitude 9.0 (visually) and a good way below this limit. Probably as far as to 10m. A third great Durchmusterung has for some time been in preparation at Cordoba in Argentina.[10] It continues the southern zones of SCHÖNFELD and is for the present completed up to 62° southern declination.