Scientific American Supplement, No. 803, May 23, 1891

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.
FIG 3.—DIAGRAM SHOWING THE COURSE OF A LUMINOUS RAY IN THE GREAT EQUATORIAL.
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.