To insure this, a positive
adjustment was provided. The end of the shaft is clamped in a
short block of wood by means of a bearing like the ones described.
One end of the block is hinged to the axis frame, while the other
end is attached by two screws, one drawing them together, the
other holding them apart. The axis is adjusted by turning these
screws. Each shaft, save the one in the pipe, is provided with
this adjustment.
The pointer is of two very thin strips placed at right angles and
tapered slightly at each end. The clamp is attached as shown in
the illustration. The eye piece is a black iron washer supported
on a small strip of wood. The aperture should be 1/4 in., since
the pupil of the eye dilates very much in darkness. The error due
to large aperture is reduced by using a very long pointer which
also makes it possible to focus the eye upon the front sight and
the star simultaneously. The forward sight is a bright brass peg
illuminated by a tiny electric lamp with a reflector to shield the
eye. The pointer arranged in this way is a great improvement over
the hollow tube sometimes used, since it allows an unobstructed
view of the heavens while indicating the exact point in question.
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