New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1899-08-21
A GIGANTIC TELESCOPE.
IMMENSE INSTRUMENT. PLANNED FOR
THE PARIS EXPOSITION OF 1900.

.r,o merely vague ideas have been formed in re-
gard 20 this great siderOstat which is of such
deep interest TO the scientific world. but last
Sunday the writer CHANGED 10 meet NJ. Eugene
artoniadi 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 description Of this marvellous in
strument.
II was at the initiative OF NJ. FRANCOIS De-
loncle Minister Plenipotentiary in the French
Diplomatic Service, that group Of amateur
tronomers decided TO devise for the International
Exhibition an instrument Of exceptional dimen-
stems and power. far exceeding anything before
attempted. With this end in view, II
determined. to give the object glass a
diameter Of 125 metres. or 49.2 inches
that is, 90 inches more than that of the
celebrated Yerkes glass at williams Bay,
Wisconsin, and 1872 inches larger than that Of
the Lick telescope, at Mount Hamilton, Cali
fornia. In order 10 utilize such an aperture TO
the best advantage, and especially TO check as
far as possible the obnoxious effect Of chro-
matte alteration, II was decided not to subord-
nate, as usual the optical work TO mechanical
difficulties by reduction Of the focal distance,
but boldly 10 give the tube the enormous length
OF sixty metres or nearly two hundred feet.
To mount such instrument on an ordinary
equatorial foot would be practically impossible,
for. 10 say nothing Of the tremendous weight OF
the tube and the consequent instability and fex-
ures 20 which IL would be exposed. the dome des-
tined 10 protect II ought TO have diameter OF
at least 30 feet or 20 . Z larger than the
cupola OF 81. Peters. in Rome, and lO3 feet more
than the dome Of st. Sophia, at Constantinople.
But this is not all. Owing 10 the apparent
d!urnal swing OF the heavens around the polar
star. the dome ought during observation TO he in
constant motion, so as 20 have its opening al
ways in front Of the object glass, moving with
velocity Of ffty-three feet an hour. The eye
piece would. Of course, also move at corre-
spending pace and it is obvious that the aero
batic feats the observer would have 10 accom-
plish in order 10 follow the movement Of the in
strument would better suit vigorous athiete
than the delicate frame Of the man Of science.
sIDEROsTAT MOUNTING.
These difficulties have been surmounted by the
wise conclusion Of the committee to adopt the
siderostat type Of mounting such as has been
perfected by the French physicist. NJ. Leon Fou-
cault a man Of remarkable mechanical genius
NJ Antoniadi uses the expression perfected by
Foucault" advisedly because he points out that
the principle OF the sderostat was known hun
dred years ago. when clever London optlciah
named Brown constructed "a telescope whose
tube was always horizontal and in which
plane mirror reflected the image Of the object TO
The eyepiece" The sderostat thus consists OF
q fat mirror. so mounted that when clockwork
motion is applied 10 it 1t will send in the same
fixed direction the rays impinging upon 1t from
q heavenly body. A telescope directed along the
reflected beam will then enable the observer TO
scrutinize the object without troubling himself
about the motion.
The fixed tube Of the great Paris reflector is Of
steel very slightly less than Oh inch thick and
weighs some twenty-one tons. Its diameter is
aftv-nine inches The cylinder is formed OF
twentv-four separate parts, screwed together
ard rests on eight cast iron supports, placed on
eight stone pillars In order 20 facilitate eXpan-
sion by heat, the supports can glide on system
Of rails attached 20 the piers
There are two object glasses. the one for VS.
ual observations. the other being reserved Tor
photographic work Each glass weighs looo
pounds They are both mounted on truck
gliding along q railway, thus allowing OF their
easy transfer in front OF the tube. The eyeplece
is also movable on a railway. and the focusing
Is effected by screw sixty inches long unltlng
the two tubes Should the mirror Of the sldero-
stat not perfectly follow the object under scrut-
my. then the corrections IN right ascension ana
decnation can be made without difnculty from
The evepiece end by most ingenious contriv-
ance The sderostat proper, which weighs some
torty-nve tons, consists Of huge brass foot
measuring twenty-six feet In length ana as
much in height and resting on a marble pier.
The diameter OF the great mirror is Ts34 inches,
mr. rather more than six and a half feet, and us
,vaisht mounting included. more than six and
half tons II is held in equilibrium by system
Of levers and counterpoises, rolling in a well
more than six and half feet in diameter, filled
with mercury The mechanical part Of the in
strlment was made by the celebrated Paris
maker. NJ. Gautier; the lenses by NJ. Mantois.
IL was no easy task 10 grind and polish the
surfaces OF the colossal mirror and or the Two
obiect glasses Here new methods had 10 be
devised The plane figure OF the mirror has been
ob,ainad nv rhe mOlar action Of two fiat metallic
sIiSers NJ Mantois used the same process IN
grinding the object glasses, with this difference.
however. that. owing 10 The curved surfaces %
be given 20 the lenses. the sliders' instead OF
being straight. had the curvature OF the ulsks'
The rectilinea motion Of the system thus gave
rise 10 a cylindrical section on the glass
which. however, in virtue or the revolution OF
the lenses on their axis was ti@4491O4 i''N ''''
spherical surface. NJ, -,
The light grasping power OF me faris tel-
escope as compared with that ol the host pow
erfu instrument now in existence 'lhe ieIkes
9lsss1 ought 20 be as three is 10 TWO. But thls
will not be the case. owing chiefly 10 the pres
ence Of the glass mirror OF the slues %
q vertical incidence. mercury ltsell Goes nor re-
fleet more than 81 per cent OF the incident hght
and here lies the gain in favor OF the ferKes
telescope The accurate hgurlns' moreover ot
disks Of such enormous size as those OF the siedt
French telescope is beset with formidabie II not
quite insuperable, difficulties, and VIZ nave some
reasons for doubling that the opticai surfaces
will be a success. Another point WhIch WIZ teIl
haaviIy against the performance OF the giant
viii be its rather dsadvantageous location IN me
midst OF a vast industrial city ana at a hele CIV
OF barely one hundred and nifty teet aOoVe .lo
.ea Level contrasting in these particulars 80 IN
fsvorablv with the pure air. serene SKIES anu
high attitudes OF our great American ooseFH a
tories We are apt 20 forget somewhat 100 Iedu
uv thai We are actually living In the bottom OF
3 denSs aerial ocean in which currents or Vari-