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THE GREAT EQUATORIAL OF THE PARIS OBSERVATORY.


The great instrument which has just completed the installation of our
national observatory is constructed upon the same principle as the
elbowed equatorial, 11 in. in diameter, established in 1882, according
to the ingenious arrangement devised as long ago as 1872, by Mr.
Loewy, assistant director of the Paris Observatory.

We shall here recall the fact that the elbowed equatorial consists of
two parts joined at right angles. One of these is directed according
to the axis of the world, and is capable of revolving around its own
axis, and the other, which is at right angles to it, is capable of
describing around the first a plane representing the celestial
equator. At the apex of the right angle there is a plane mirror of
silvered glass inclined at an angle of 45 deg. with respect to the
optical axis, and which sends toward the ocular the image coming from
the objective and already reflected by another and similar plane
mirror. 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