PHYSICAL ASTRONOMY. 161 
as much as _ possible the weight resting on the beds of the horizontal axis : 
this is done by counterpoises, as seen at H, H (fig. 13), L, L (fig. 11), and 
G, G (fig.3). These act on one arm of levers whose fulcra are supported 
by the solid parts of the instrument, the other arms carrying stirrups 
through which the axis is received, so that this latter just touches the socket 
in which its pivots turn. 
In the interior of the telescope, at the focus of the eye-glass, is the wire 
plate of fine threads of wire or other material, of which two are horizontal 
and an indefinite number vertical. The object here is to give greater 
precision to observation, by dividing the field of view into a certain number 
of subdivisions. This wire plate can be so moved as to be brought accu- 
rately into focus, and there regulated; to render it visible, however, it must 
be illuminated from without. This is done by making one half of the axis 
hollow, and reflecting the light of a lamp through this cavity into a hole in 
the side of the tube of the telescope. By this means, the cross lines are 
dark on a light ground. The illumination employed by Fraunhofer is, how- 
ever, much more convenient; this is applied between the eye of the observer 
and the focus. Here the whole field of the telescope remains dark, the cross 
lines alone being illuminated, so that object and cross lines can be distinctly 
seen at the same time. As much depends upon the perfectly horizontal 
position of the axis of the instrument, it becomes necessary to apply frequently 
a test of this, which is done by a tubular level placed above or below, as in 
pl. 15, fig. 13, L. In the portable transit instrument (pl. 15, fig. 22), the 
stand, AABBCC, consists of a cast iron crown, upon which the two parts 
for the axis are immovably fastened. The horizontal position of the instru- 
ment is controlled in one direction by a level placed upon the axis; in the 
other, by a level, F, placed upon the declination circle, D. 
The rectification of an astronomical instrument, or the determination of 
its faults, must precede its use. Three principal errors may attach to the 
‘ransit instrument: in the first place, it will almost always deviate from the 
meridian, that is, it will have a small eastern or western azimuth; in the 
second place, it will almost always be inclined at a small angle to the plane 
of the horizon, which is determined by the dependent level, L (fig. 18); 
thirdly, the optical axis of the telescope will deviate at a slight angle from 
the line drawn perpendicular to the axis of rotation: this last error is called 
the error of collimation. These three errors must be ascertained and recti- 
fied, partly mechanically, and partly by calculation; as also the four errors 
to which the diaphragm or wire plate is generally subject. 
Fig. 25 is a side, and fig. 31 a back view of a small transit instrament 
constructed by Repsold for the observatory of St. Petersburgh. <A are the 
pillars of the axis G, fastened to a granite block: the pieces in which the 
pivots of the axis turn, are shown more in detail by figs. 26 and 27. E is 
the declination circle with its vernier, F, and the level, I, situated above the 
axis. The telescope, BD, is only partly given. At Dis the adjusting screw 
for the ocular, given more in detail by figs. 28 and 29. Its micrometer 
arrangement is shown in fig. 30, where the wire plate is move! in the box, 
b, by the screw a. Fig. 32 is one of two supports which sta::.| in excava- 
ICONOGRAPHIC ENCYCLOPZDIA.—VOL. I. 11 161 
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