Scientific American Supplement, No. 803, May 23, 1891 | Page 2

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