roduction 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 Abo,
then in Haelsingfors, 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 deg..

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 deg. in declination. The assistant sits in the room
below, separated by a board floor, at the _Thiede_ clock.

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 deg.
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 deg..174, which
signifies: Zone +23 deg., star No. 174.

ARGELANDER's work was continued for stars between [del