PHYSICAL ASTRONOMY. 165. 
similar to that of the Dorpat refractor. One of this character, for example, 
is to be found in Munich, with a telescope of 8 feet focus and 6 inches 
aperture; the hour circle has a diameter of 9 inches, graduated to 4 
seconds of time; and the declination circle, a diameter of 12 inches, divided 
to arcs of 10 seconds. The telescope, admitting a magnifying power of 400, 
follows, by means of a clock with a centrifugal pendulum, the diurnal motion 
of the stars. 
The Theodolite. 
90. Another instrument to be noticed in this place is the Theodolite. 
Fig. 35 represents a lateral, fig. 36 an edge, and fig. 837 a superior view of 
a Theodolite constructed by Ertel of Munich. In the three views the same 
letters refer to the same parts. It rests upon a tripod stand, AA, with three 
adjusting screws, of which only two, B, B, are represented. On this tripod 
is a short column, C, and upon this column is placed the horizontal circle E, 
graduated to degrees, &c. Upon this, and turning on its centre, the stand- 
ards, H, H, rotate. These carry at their extremities, pivot holes for the 
horizontal axis of the telescope, N, whose optical axis moves in a vertical 
plane. The whole arrangement is similar to that of a transit instrument, 
with this difference, that the Theodolite has a very finely graduated vertical 
circle for measuring altitudes, while the horizontal circle, E, is intended to 
measure horizontal angles, which are read off by the lenses, G. This hori- 
zontal circle, E, can be fastened or loosened at pleasure by the clamp 
arrangement, Fabb. K, K, are lenses or microscopes for reading off the 
vertical angles measured on the circle L, and M is the level required for 
rectifying the station of the instrument. 
Theodolites are divided into two principal kinds—Compensating and 
Repeating Theodolites ; they are also provided sometimes with a so-called 
rectifying telescope. As regards the use of the instrument, we would refer 
to what has been said of it in the mathematical portion of the work. 
The errors and rectifications of the Theodolite are much the same as 
those of the meridian and transit instruments (sections 84 and 87). In 
conclusion, it may be remarked, that for the Theodolite may be substituted 
a repeating cirele, a simple circle, or an universal instrument, as constructed 
by Ertel of Munich, and A. and G. Repsold in Hamburg. These instru- 
ments fulfil the aim of the Theodolite just as well, and even more 
completely ; at least this is the case as far as regards astronomical 
observations. 
The Reflecting Sextant ; the Reflecting Sector; the Triquetrum. 
91. Atl the instruments already mentioned, as used for measuring angles, 
require an immovably vertical or horizontal position. This, however, 
cannot always be attained, in which case reflecting instruments are 
165 
