Astronomical and Nautical Instruments. 
The following is an enumeration and brief description of the instruments 
used for astronomical observations. 
Altazimuth by C. H. G. Otsen in Christiania. The horizontal and vertical 
circles, of 21 cm. diameter, are graduated on platinum to 10’ and read off 
by two microscopes placed diametrically reading to 10”, the single seconds 
being easily estimated. The microscopes for the horizontal circle follow the 
instrument in the motion about the vertical axis, the horizontal circle being 
fixed, while the vertical circle follows the telescope in the motion about the 
horizontal axis, the corresponding microscopes being fixed at the ends of the 
horizontal diameter. The alidade bearing these microscopes is provided with 
a fixed level, adjustable by screw and spring working on an arm going 
downwards from the centre. Both circles being graduated from left to right, 
as seen from the centre, the correction to the reading of the vertical circle 
is positive when the right end of the level is the higher. The level is divided 
from the middle and the angular value of a part was given by the maker as 
4”, which was found to be sufficiently accurate; consequently the difference 
between the readings of the two ends of the bubble, multiplied by 2, gives 
the correction to the circle-reading in seconds. The degrees and tens of 
minutes are for both circles read off by an index, which is, for the vertical 
circle, placed at the top. 
The microscopes of both circles were always pointed to two adjacent 
division lines, one on each side of the central notch marking the zero of 
single minutes in the field of the microscope. Generally the two readings 
did not differ by more than the accidental error of pointing, a few seconds; 
