Sky and Telescope - July 2017 - 33

light in 1917 and opened the way for 20th-century astronomy.
About 50 million visitors came to the Paris exhibition. The
public lined up to see new types of electric lighting and appliances, talking movies, escalators, and diesel engines as well as
the giant telescope. But many astronomers were skeptical of
its unorthodox design, particularly after reading exaggerated
press reports that it could resolve one-meter objects on the
Moon! Deloncle himself tested the telescope on the ﬁrst clear
night when the Moon was observable with the siderostat. He
wrote, "On the square ground-glass plate before our eyes [in
the plateholder] the moon's image gradually crept up from
one corner until it had overspread the glass completely. And
there we stood in the centre of Paris examining the surface of
our satellite, with all its craters and valleys and bleak desolation! I had won the day!"
Charles P. Butler was quite impressed after an observing
session arranged by Deloncle. Even during the exhibition,
he wrote in Nature (Oct. 11, 1900), the telescope was being
used for astronomy every clear night. Opening the roof took
six to eight people, he reported, but only two were needed
for observations: one adjusting the siderostat and the other
adjusting the optics at the eyepiece end. The two communicated over a telephone line. Considering that even the
lowest-power eyepiece gave 500× and a ﬁeld of view only 3
arcminutes wide, Butler wrote "it is astonishing how quickly
an object is obtained after the setting of the circles" on the
siderostat. Once the object was found, interior lights were
turned off for photography or viewing. The siderostat tracked
so well that an object would stay in view for 45 minutes.
Butler noted that the Ring Nebula in Lyra, M57, looked
better in the telescope than in a conventional 36-inch. It also
was bright enough for detail to be seen in the highly magniﬁed image. Observations were possible even with hundreds
of people in the building and bright outdoor lighting on
adjacent buildings. At midnight when public viewing ended,
Eugène Antoniadi of Juvisy Observatory would take over to
observe nebulae. Butler praised both the optical quality and
the clock drive of the great telescope and hoped that after
the exhibit, it would ﬁnd a new home "at some station out of
the city, where the purity of the atmosphere will allow of its
power being efﬁciently used."
A few other astronomers reported making observations.
Antoniadi found that the refractor's large aperture made it
"wonderfully efﬁcient on nebulae." He also observed Venus.
Charles Le Morvan took several photos of the Moon that
nearly ﬁlled glass plates two feet square. Théophile Moreux
sketched the ﬁne details of a sunspot seen by projection.
But overall, the great refractor's scientiﬁc output was
suspiciously slight. Everyone who used it expected that the
seeing would be much better outside the city. But the image
quality must surely have been affected by the light passing
through 200 feet of steel tubing with no provision for ventilation to prevent slight air-temperature differences ("tube
currents," a bane of high-power observing) along the way.
To Flammarion, the telescope's success was further evi-

dence that "the future lies in the development of refracting
telescopes rather than reﬂecting." Yet that was not to be.
Ticket sales at the exposition failed to recover the Société
d'Optique's investment. The company put the telescope up
for sale after the exposition closed, but no one bought it.
Most astronomers remained skeptical of the design and its
limitations; for one thing, it could view only part of the sky.
It had been built for exhibition rather than for observatory
use, so changes would be required to meet astronomers'
needs, and success may have seemed uncertain. Then there
was the instrument's sheer size. It was one thing to build a
world's-fair exhibit for paying viewers unaware perhaps of
subtle problems, but another to retool such a giant instrument for professional needs in a working observatory without breaking the bank.
The telescope remained at the exposition site until 1909,
when both the Société d'Optique and Gautier's optical ﬁrm
ﬁled for bankruptcy.
No buyer for the instrument came to the bankruptcy
auction. So the tube and other mechanical parts were sold
as scrap. The siderostat mirror went to Paris Observatory,
where it has long been displayed in the history building. The
ﬁnished ﬂint and crown elements of the photographic objective went to the observatory cellars, where they lay forgotten
in wooden packing boxes until about 2002 when Françoise
Launay of the Paris Observatory helped unearth them. The
fate of the visual objective remains unknown.
More than a century after it was disassembled, it's hard
to assess the great refractor's actual performance. The heavy
lenses have not been studied in detail with modern instruments, but Launay wrote in the Journal of Historical Astronomy
(p. 459, vol. 38, 2007) that the glass looks clear and shows few
bubbles. Although the instrument's performance suffered from
its location, she says that observations by Antoniadi and a
handful of others show that it indeed had astronomical value.

Perhaps the limits of a technology can
be found only by exceeding them.
The Paris telescope proved a technological dead end, the
swan song for large refractors. Perhaps the limits of a technology can be found only by exceeding them. By the time it
was taken apart in 1909, George Ellery Hale already had a
60-inch reﬂector operating successfully on Mount Wilson
and had begun on the 100-inch - which opened the age of
giant reﬂectors, with never another look back.
¢ JEFF HECHT, a Fellow of the Optical Society, covers optics
and lasers for magazines including New Scientist and Optics
& Photonics News. His books include Understanding Lasers
and City of Light: The Story of Fiber Optics. His father gave
him his first telescope when he was 10, and he's been fascinated by light and the sky ever since.
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Sky and Telescope - July 2017

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