He considered the concave gratings to
make a new departure in the subject, and that they would have greatly
facilitated the preparation of his map.
* * * * *
A NEW POCKET OPERA GLASS.
[Illustration: POCKET OPERA GLASS.]
Inasmuch as high power combined with small size is usually required in
an opera glass, manufacturers have always striven to unite these two
features in their instruments, and have succeeded in producing glasses
which, although sufficiently small to be carried in the waistcoat
pocket, are nevertheless powerful enough to allow quite distant objects
to be clearly distinguished. Recently, a Parisian optician has succeeded
in constructing an instrument of this kind that is somewhat of a novelty
in its way, since its mechanism allows it to be closed in such a manner
as to take up no more space than a package of cigarettes (Fig. 1.) It is
constructed as follows:
AB and CD (Fig. 1) are two metallic tubes, in which slide with slight
friction two other tubes. Into the upper part of the latter are inserted
two hollow elliptical eye-pieces, which move therein with slight
friction, and which are united by the two supports tor the wheel, _bb_
(Fig. 4), and endless screw that serve for focusing the instrument. The
eyepieces, TT, are held in the tube by means of two screws, _vv_ (Figs.
2 and 4), in such a way that they can revolve around the latter as axes.
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