[Footnote 4: In order to deduce from _M_ the apparent magnitude at a distance corresponding to a parallax of 1″ we may subtract 3m.48. To obtain the magnitude corresponding to a parallax of 0″.1 we may add 1.57. The latter distance is chosen by some writers on stellar statistics.] [Footnote 5: The best colour-scale of the latter sort seems to be that of OSTHOFF.] [Footnote 6: Compare H. A. 50 and H. A. 56 and the remarks in L. M. II, 19.] CHAPTER II. SOURCES OF OUR PRESENT KNOWLEDGE OF THE STARS. 18. In this chapter I shall give a short account of the publications in which the most complete information on the attributes of the stars may be obtained, with short notices of the contents and genesis of these publications. It is, however, not my intention to give a history of these researches. We shall consider more particularly the questions relating to the position of the stars, their motion, magnitude, and spectra. 19. _Place of the stars._ _Durchmusterungs._ The most complete data on the position of the stars are obtained from the star catalogues known as “Durchmusterungs”. There are two such catalogues, which together cover the whole sky, one--visual--performed in Bonn and called the _Bonner Durchmusterung_ (B. D.), the other--photographic--performed in Cape _The Cape Photographic Durchmusterung_ (C. P. D.). As the first of these catalogues has long been--and is to some extent even now--our principal source for the study of the sky and is moreover the first enterprise of this kind, I shall give a somewhat detailed account of its origin and contents, as related by ARGELANDER in the introduction to the B. D. B. D. was planned and performed by the Swedish-Finn ARGELANDER (born in Memel 1799). A scholar of BESSEL he was first called as director in Åbo, then in Hälsingfors, and from there went in 1836 to Bonn, where in the years 1852 to 1856 he performed this great _Durchmusterung_. As instrument he used a FRAUENHOFER comet-seeker with an aperture of 76 mm, a focal length of 650 mm, and 10 times magnifying power. The field of sight had an extension of 6°. In the focus of the objective was a semicircular piece of thin glass, with the edge (= the diameter of the semicircle) parallel to the circle of declination. This edge was sharply ground, so that it formed a narrow dark line perceptible at star illumination. Perpendicular to this diameter (the “hour-line”) were 10 lines, at each side of a middle line, drawn at a distance of 7′. These lines were drawn with black oil colour on the glass. The observations are performed by the observer A and his assistant B. A is in a dark room, lies on a chair having the eye at the ocular and can easily look over 2° in declination. The assistant sits in the room below, separated by a board floor, at the _Thiede_ clock.