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.