The objective and this second mirror (which is inclined at an
angle of 45 deg.) are placed at the extremity of the external part of
the tube, and form part of a cube, movable around the axis of the
instrument at right angles with the axis of the world. The diagram in
Fig. 3 will allow the course of a luminous ray coming from space to be
easily understood. The image of the star, A, toward which the
instrument is directed, traverses the objective, B C, is reflected
first from the mirror, B D, and next from the central mirror, E F, and
finally reaches O, at the ocular where the observer is stationed.
This new equatorial differs from the first model by its much larger
dimensions and its extremely remarkable mechanical improvements. The
optical part, which is admirably elaborated, consists of a large
astronomical objective 24 in. in diameter, and of a photographic
objective of the same aperture, capable of being substituted, one for
the other, according to the nature of the work that it is desired to
accomplish by the aid of this colossal telescope, the total length of
which is 59 ft. The two plane mirrors which complete the optical
system have, respectively, diameters of 34 in. and 29 in. These two
magnificent objectives and the two mirrors were constructed by the
Brothers Henry, whose double reputation as astronomers and opticians
is so universally established.
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