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
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. The mechanical part is the successful
work of Mr. Gautier, who has looked after every detail with the
greatest care, and has thus realized a true _chef d'oeuvre_. The
colossal instrument, the total weight of which is 26,400 lb., is
maneuvered by hand with the greatest ease. A clockwork movement, due
to the same able manufacturer, is capable, besides, of moving the
instrument with all the precision desirable, and of permitting it to
follow the stars in their travel across the heavens. A star appearing
in the horizon can thus be observed from its rising to its setting.
The astronomer, his eye at the ocular, is always conveniently seated
at the same place, observing the distant worlds, rendered immovable,
so to speak, in the field of the instrument. For stars which, like the
moon and the planets, have a course different from the diurnal motion,
it is possible to modify the running of the clockwork, so that they
can thus be as easily followed as in the preceding case. Fig. 1 gives
a general view of the new installation, for which it became necessary
to build a special edifice 65 ft. in height on the ground south of the
observatory bordering on the Arago Boulevard. A large movable
structure serves for covering the external part of the instrument.
This structure rests on rails, upon which it slides toward the south
when it is desired to make observations. It will be seen from the
figure how the principal axis of the instrument rests upon the two
masonry pillars, one of which is 49 ft. and the other 13 ft. in
height.
[Illustration: FIG 1.--THE GREAT EQUATORIAL OF THE PARIS OBSERVATORY.]
The total cost of the pavilion, rolling structure, and instrument
(including the two objectives) will amount to about $80,000 after the
new equatorial has been provided with the scientific apparatus that
necessarily have to accompany it for the various and numerous
applications to which the use of it will give rise.
[Illustration: FIG 2.--OCULAR OF THE GREAT EQUATORIAL.]
Fig. 2 shows us the room in the observatory in which the astronomer,
seated in his chair, is completely protected against the inclemencies
of the weather. Here, with his eye applied to the ocular, he can,
without changing position (owing to all the handles that act at his
will upon the many transmissions necessary for the maneuvering),
direct his instrument unaided toward every point of the heavens with
wonderful sureness and precision. The observer has before him on the
same plane two divided circles, one of which gives the right
ascensions and the other the declinations, and which he consults at
each observation for the exact orientation of the equatorial.
[Illustration: FIG. 3.--DIAGRAM SHOWING THE COURSE OF A LUMINOUS RAY
IN THE GREAT EQUATORIAL.]
All the readings are done by the aid of electric lamps of very small
dimensions, supplied by accumulators, and which are lighted at will.
Each of these lamps is of one candle power; two of them are designed
for the reading of the two circles of right ascension and of
declination; a third serves for the reading of the position circle of
the micrometer; two others are employed for the reading of the drums
fixed upon the micrometric screws; four others serve for rendering the
spider threads of the reticule brilliant upon a black ground; and
still another serves for illuminating the field of the instrument
where the same threads remain black upon a luminous ground. The
currents that supply these lamps are brought over two different
circuits, in which are interposed rheostats that permit of graduating
the intensity of the light at will.
Since the installation of the first model of an elbowed equatorial of
11 in. aperture, in 1882, at the Paris Observatory, the numerous and
indisputable advantages of this sort of instrument have led a certain
number of observatories to have similar, but larger, instruments
constructed. In France, the observatories of Alger, Besancon, and
Lyons have telescopes of this kind, the objectives of which have
diameters of from 12 in. to 13 in., and which have been used for
several years past in equatorial observations of all kinds. The Vienna
Observatory has for the last two years been using an instrument of
this kind whose objective has an aperture of 15 inches. Another
equatorial of the same kind, of 16 in. aperture, is now in course of
construction for the Nice Observatory, where it will be especially
employed as a seeker of exceptional power--a role to which this kind
of instrument lends itself admirably. The optical part of all these
instruments was furnished by the Messrs. Henry, and the mechanical
part by Mr. Gautier.
The largest elbowed equatorial is, therefore, that of the Paris
Observatory. Its optical power, moreover, corresponds perfectly to its
huge dimensions. The experimental observations which have already been
made with it fully justify the hopes that we had a right to found upon
the professional skill of the eminent artists to whom we owe this
colossal instrument. The images of the stars were given with the
greatest sharpness, and it was possible to study the details of the
surface of the moon and other planets, and several star clusters, in
all their peculiarities, in the most remarkable manner.
When it shall become possible to make use of this equatorial for
celestial photography, there is no doubt that we shall obtain the most
important results. As regards the moon, in particular, the
photographing of which has already made so great progress, its direct
image at the focus of the large 24 in. photographic objective will
have a diameter of 11 in., and, being magnified, will be capable of
giving images of more than 3 ft. in diameter.--_La Nature_.
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