The evening times., 1899-09-26
4 GIGANTIC TELESCOPE
Immense Instrument Planned for
the Paris Exposition.
1t VIII BE One of the Most RemaE1E-
able Features of the Coming I'm
Explorations of the Heavens VIII
BE Made With This Monster Glass.
(From the New York Tribune. J

gigantic telescope with which astronomers
in 1900 and succeeding years will explore
the heavens. Hitherto merely vague ideas
have been formed in regard to this great
siderostat which is OF such deep interest to
the scientific world, but last Sunday the
writer chanced to meet NJ. Eugene Anto-
niadi. assistant astronomer of the Juvisy
Observatory, who with great courtesy
placed at the disposal of the. readers OF
the Tribune" such facts and information
as will present precise and accurate de-
scription of this marvellous instrument.
1t was at the initiative of M. Francois
Deloncle Minister Plenipotentiary in the
French diplomatic service, that group OF
amateur astronomers decided to devise for
the International Exhibition an instrument
of exceptional dimensions and power, far
exceeding anything before attempted. With
this end in view, 1t was determined to give
the object glass diameter Of 1.25 metres,
or 1872 inches-that is, SO inches more
than that of the celebrated Yerkes glass at
Williams Bay, Wis, and 1872 inches larger
than that of the Lick telescope. at Mount
Hamilton, Cal. In order to utilize such an
aperture to the best advantage, and espe-
cially to check as far as possible the OBJ
noxious effect of chromatic alteration, IT
was decided not to subordinate, as usual,
the optical work to mechanical difficulties
boldly to give the tube the enormous length
of 60 metres, or nearly 200 feet.
To mount such an instrument on an or
binary equatorial foot would be pract caliy
impossible, for, to say nothing OF the tre-
mendous weight of the tube and the conse-
uuent instability and fieXures to which 1t
would be exposed, the dome destined 10
protect it ought to have a diameter of at
least 210 feet, or 72 feet larger than the
cupola of St. Peters, in Rome. and 103
feet more than the dome of st. Sophia, at
Constantinople But this is not all. Ow
ing to the apparent diurnai swing Of the
heavens around the polar star, the dome
ought during observation to be In constant
motion, so as to have its opening always
in front of the object glass, moving with
velocity of fifty-three feet an hour The
eyepiece would. of course, a so move at
a corresponding pace. and it Is obvious
that the acrobatic feats the observer
would have 10 accomplish In order 10 fol-
low the movement of the instrument would
better suit a vigorous athlete than the
delicate frame of the man OF science.
These difficulties have been surmoanted
by the wise conclusion of the committee
to adopt the siderostat type OF mounting.
such as has been perfected by the Frencn
physicist, M. Leon Foucault, man OF re-
markable mechanical genius. NJ. An .onia-
di uses the expression perfected by Fol-
cault'' advisably, because he points out
that the principle OF the side. os;at was
known a hundred years ago, when clever
London optician named Brown construct
ed "a telescope whose tube was always
horizontal. and in which a plane mirror
reflected the image of the object to the
eyepiece" The siderostat thus consists o,
a fat mirror. so mounted that when clock
work motion is applied to it 1t wi ] send
in the same fixed direction the rays i'm
pinging upon it from a heavenly body. A
telescope directed along the reflected
beam will then enable the observer to
scrutiniZe the object without tro Ibling
himself about the motion.
The fixed tube Of the great Paris refiec-
tor is of steel, very slightly less than OIL
inch thick, and weighs some twenty-one
tons. Its diameter is fiftynine inches. The
cylinder is formed of twenty-four separ-
ate parts, screwed together, and rests on
eight cast-iron supports. placed on eight
stone pillars. In order to facilitate ex-
pansion by heat, the supports can glide on
a system of rails attached TO the piers.
There are two object glasses, the one for
visual observations, the other being reserv-
ed for Photographic work. Each glass
weighs 600 pounds. They are both mount
ed on a truck gliding along a railway, thus
allowing Of their easy transfer in front of
the tube. The eyepiece is also movable
on railway, and the focusing is effected
by screw sixty inches long, uniting the
two tubes. Should the mirror Of the sid
erostate not perfectly follow the object un-
der scrutiny, then the corrections in right
ascension and declination can be made
without difficulty from the eyepece end by
most ingenious contriyance. The sidero-
stat proper, which weighs some forty-five
tons, consists of a huge brass foot, meas
uring twenty-six feet in length and as
much in height, and resting on marble
pier. The diameter of the great mirror is
seventy eight and three-quarter inches. or
rather more than six and one-half feet, and
its weight, mounting included. more than
six and half tons 1t is held in equili-
brium by system of levers and counter
poises, rolling in a well more than six and
half feet in diameter, filled with mercury.
The mechanical part of the instrument was
made by the celebrated Paris maker, NJ.
Gautier; the lenses by M. Mantois.
1t was no easy task to grind and polish
the surfaces of the colossal mirror and of
the two object glasses. Here new methods
had to be devised. The plane figure of the
mirror has been obtained by the molar ac-
tion Of two fiat metallic sliders. NJ Man
tots used the same process in grinding the
object glasses, with this difference how
ever, that, owing to the curved surfaces to
be given to the lenses, the sliders, instead
Of being straight, had the curvature of the
disks The rectilineal motion of the sys-
tenn thus gave rise to a cylindrical section
on the glass which, however, in virtue of
the revolution of the lenses on their axis,
was transformed into spherical surface
The light-grasping power of the Paris
telescope, as compared with that of the
most powerful instrument now in exist
ence the Yerkes glass; ought to be as
three to two. But this will not be the
case, owing chiefly to the presence of the
glass mirror of the siderostat. Under a
vertical incidence, mercury itself does not
reflect more than 81 per cent of the Incl
dent light, and here lies the gain in favor
Of the Yerkes telescope. The accurate fig
uring, moreover, Of disks of such enormous
size as those OF the great French telescope
is beset with formidable, IL not quite in
superable, difficulties, and we have some
reasons for doubling that the optical sur
faces will be success. Another point
which will tell heavily against the per-
formance of the giant will be its rather
disadyantageous location in the midst of a
vast industrial city and at a height of
barely $50 feet above the sea level con-
trasting in these particulars so unfavora-
bly, with the pure air, serene skies and
high attitudes of our great American OBJ
servatories. We are apt to forget some
what too readily that We are actually living
in the bottom of the dense aerial ocean, in
which currents of various temperatures and
densities are continually streaming in all
directions. The greater tho haipht uo r;c IN