[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