Fig. 3.
The Telescope, being balanced on trunnions, can be moved from an
elevation approaching the zenith to an almost horizontal position.
In order that it may be secured anywhere between these extremes,
attached to the upper part of the telescope is an iron rod, which,
sliding through the end of the arm of the stand, can be there
clamped. The telescope will now be clamped in _Altitude_. As the
progression of celestial objects will apparently be very slow,
resource must be had to the smoothness of motion obtained by a
screw. The upper end of the Altitude rod is therefore tapped to
receive a long screw with a large milled head, jointed to the
telescope body; by revolving this head the telescope is raised or
depressed accordingly as the screw is unscrewed, or the reverse. It
is necessary that the screw should be withdrawn some way from the
rod before clamping it, preparatory to following an object which has
passed the meridian, or is setting; as perhaps, just when the
clearest vision is obtained, the observer may be annoyed by the
screw action being suddenly stopped by the milled head coming in
contact with the top of the rod.
When viewing objects near the zenith, and the focus long, the handle
attached to the clamp will be found useful, as it can thereby be
reached without leaving the finder. The handle may be so placed that
a downward push should clamp, and an upward pull release.
The second motion in the Alt-azimuth Stand, namely, _Azimuth_, is
obtained as follows:—The strong iron disc which forms the upper
fitting of the legs has its surface accurately turned. On this
revolves an iron disc, rather less in diameter, to which the
trunnions which support the telescope are attached. The main axis of
this disc passes through the centre of the lower disc, and then
through a hollow bearing tube, a continuation of it. All these
fittings having been most carefully turned and ground together,
great steadiness, combined with facility of horizontal movement, is
ensured. In order that this motion can be communicated as evenly as
possible, resource must be again had to a screw which is thus
applied. Just within the circumference of the lower disc is a narrow
groove, turned to such a depth that the ring which is thus separated
from the main disc is still firmly held to it by the uncut portion.
An iron clamp grooved to this ring holds the nut of a long screw,
the plain end of which is jointed to the upper disc. When this clamp
is fixed to the ring, any motion given to the screw will act on the
upper disc, and cause it to revolve, and thus the whole telescope
will be slowly moved in _Azimuth_. The advantages of this plan are
many, the most important being the rapidity and ease with which the
telescope can be shifted from one object to another, even to those
in contrary directions; all that is necessary being to release the
clamp and turn the telescope to the object required. The clamp being
carried round with the upper disc, can be fixed directly the desired
position is obtained, when the screw is at once in action. Should it
happen, whilst following an object, that the screw becomes exhausted
from the joint and clamp coming together, the clamp should be
released, and the screw turned sufficiently in the reverse way to
bring it into action, the weight on the upper disc keeping the
telescope meanwhile in position. If this operation be rapidly
performed, the whole length of screw can be brought in play before
the object has left the field of view of the finder, and can thus be
easily refound with the higher power of the telescope. Motion is
applied to the screw by means of a Hook’s joint, named thus from its
inventor. This joint being furnished with a long handle, enables the
observer, by means of it, to move the Telescope in _Azimuth_ at any
rate, and without removing his eye from the eye-piece.
It will be seen from the preceding remarks that by means of the
vertical and horizontal screw motions, the telescope, when clamped,
can be moved in any direction with the greatest facility, permitting
a celestial object to be observed with high powers for a
considerable time, and with the greatest pleasure and comfort to the
observer.
[ Illustration: decorative ]
[ Illustration: decorative ]