THE TELESCOPE OF THE ROCKY MOUNTAINS.
On the 20th of October of the preceding year, after the subscription
list was closed, the president of the Gun Club had credited the
Cambridge Observatory with the sums necessary for the construction of a
vast optical instrument. This telescope was to be powerful enough to
render visible on the surface of the moon an object being at least nine
feet wide.
There is an important difference between a field-glass and a telescope,
which it is well to recall here. A field-glass is composed of a tube
which carries at its upper extremity a convex glass called an
object-glass, and at its lower extremity a second glass called ocular,
to which the eye of the observer is applied. The rays from the luminous
object traverse the first glass, and by refraction form an image upside
down at its focus. This image is looked at with the ocular, which
magnifies it. The tube of the field-glass is, therefore, closed at each
extremity by the object and the ocular glasses.
The telescope, on the contrary, is open at its upper extremity. The rays
from the object observed penetrate freely into it, and strike a concave
metallic mirror--that is to say, they are focussed. From thence their
reflected rays meet with a little mirror, which sends them back to the
ocular in such a way as to magnify the image produced.
Thus in field-glasses refraction plays the principal part, and
reflection does in the telescope. Hence the name of refractors given to
the former, and reflectors given to the latter. All the difficulty in
the execution of these optical instruments lies in the making of the
object-glass, whether they be made of glass or metallic mirrors.
Still at the epoch when the Gun Club made its great experiment these
instruments were singularly perfected and gave magnificent results. The
time was far distant when Galileo observed the stars with his poor
glass, which magnified seven times at the most. Since the 16th century
optical instruments had widened and lengthened in considerable
proportions, and they allowed the stellar spaces to be gauged to a depth
unknown before. Amongst the refracting instruments at work at that
period were the glass of the Poulkowa Observatory in Russia, the
object-glass of which measured 15 inches in width, that of the French
optician Lerebours, furnished with an object-glass equally large, and
lastly that of the Cambridge Observatory, furnished with an object-glass
19 inches in diameter.
Amongst telescopes, two were known of remarkable power and gigantic
dimensions. The first, constructed by Herschel, was 36 feet in length,
and had an object-glass of 4 feet 6 inches; it magnified 6,000 times;
the second, raised in Ireland, at Birrcastle, in Parsonstown Park,
belonged to Lord Rosse; the length of its tube was 48 feet and the width
of its mirror 6 feet; it magnified 6,400 times, and it had required an
immense erection of masonry on which to place the apparatus necessary
for working the instrument, which weighed 12-1/2 tons.
But it will be seen that notwithstanding these colossal dimensions the
magnifying power obtained did not exceed 6,000 times in round numbers;
now that power would only bring the moon within 39 miles, and would only
allow objects 60 feet in diameter to be perceived unless these objects
were very elongated.
Now in space they had to deal with a projectile 9 feet wide and 15 long,
so the moon had to be brought within five miles at least, and for that a
magnifying power of 48,000 times was necessary.
Such was the problem propounded to the Cambridge Observatory. They were
not to be stopped by financial difficulties, so there only remained
material difficulties.
First of all they had to choose between telescopes and field-glasses.
The latter had some advantages. With equal object-glasses they have a
greater magnifying power, because the luminous rays that traverse the
glasses lose less by absorption than the reflection on the metallic
mirror of telescopes; but the thickness that can be given to glass is
limited, for too thick it does not allow the luminous rays to pass.
Besides, the construction of these vast glasses is excessively
difficult, and demands a considerable time, measured by years.
Therefore, although images are better given by glasses, an inappreciable
advantage when the question is to observe the moon, the light of which
is simply reflected they decided to employ the telescope, which is
prompter in execution and is capable of a greater magnifying power; only
as the luminous rays lose much of their intensity by traversing the
atmosphere, the Gun Club resolved to set up the instrument on one of the
highest mountains of the Union, which would diminish the depth of the
aërial strata.
In telescopes it has been seen that the glass placed at the observer's
eye produces the magnifying power, and the object-glass which bears this
power the best is the one that has the largest diameter and the greatest
focal distance. In order to magnify 48,000 times it must be much larger
than those of Herschel and Lord Rosse. There lay the difficulty, for the
casting of these mirrors is a very delicate operation.
Happily, some years before a _savant_ of the _Institut de France_, Léon
Foucault, had just invented means by which the polishing of
object-glasses became very prompt and easy by replacing the metallic
mirror by taking a piece of glass the size required and plating it.
It was to be fixed according to the method invented by Herschel for
telescopes. In the great instrument of the astronomer at Slough, the
image of objects reflected by the mirror inclined at the bottom of the
tube was formed at the other extremity where the eyeglass was placed.
Thus the observer, instead of being placed at the lower end of the tube,
was hoisted to the upper end, and there with his eyeglass he looked down
into the enormous cylinder. This combination had the advantage of doing
away with the little mirror destined to send back the image to the
ocular glass, which thus only reflected once instead of twice; therefore
there were fewer luminous rays extinguished, the image was less feeble,
and more light was obtained, a precious advantage in the observation
that was to be made.
This being resolved upon, the work was begun. According to the
calculations of the Cambridge Observatory staff, the tube of the new
reflector was to be 280 feet long and its mirror 16 feet in diameter.
Although it was so colossal it was not comparable to the telescope
10,000 feet long which the astronomer Hooke proposed to construct some
years ago. Nevertheless the setting up of such an apparatus presented
great difficulties.
The question of its site was promptly settled. It must be upon a high
mountain, and high mountains are not numerous in the States.
In fact, the orographical system of this great country only contains two
chains of average height, amongst which flows the magnificent
Mississippi, which the Americans would call the "king of rivers" if they
admitted any royalty whatever.
On the east rise the Apalachians, the very highest point of which, in
New Hampshire, does not exceed the very moderate altitude of 5,600 feet.
On the west are, however, the Rocky Mountains, that immense chain which
begins at the Straits of Magellan, follows the west coast of South
America under the name of the Andes or Cordilleras, crosses the Isthmus
of Panama, and runs up the whole of North America to the very shores of
the Polar Sea.
These mountains are not very high, and the Alps or Himalayas would look
down upon them with disdain. In fact, their highest summit is only
10,701 feet high, whilst Mont Blanc is 14,439, and the highest summit of
the Himalayas is 26,776 feet above the level of the sea.
But as the Gun Club wished that its telescope, as well as the Columbiad,
should be set up in the States of the Union, they were obliged to be
content with the Rocky Mountains, and all the necessary material was
sent to the summit of Long's Peak in the territory of Missouri.
Neither pen nor language could relate the difficulties of every kind
that the American engineers had to overcome, and the prodigies of
audacity and skill that they accomplished. Enormous stones, massive
pieces of wrought-iron, heavy corner-clamps, and huge portions of
cylinder had to be raised with an object-glass, weighing nearly 30,000
lbs., above the line of perpetual snow for more than 10,000 feet in
height, after crossing desert prairies, impenetrable forests, fearful
rapids far from all centres of population, and in the midst of savage
regions in which every detail of life becomes an insoluble problem, and,
nevertheless, American genius triumphed over all these obstacles. Less
than a year after beginning the works in the last days of the month of
September, the gigantic reflector rose in the air to a height of 280
feet. It was hung from an enormous iron scaffolding; an ingenious
arrangement allowed it to be easily moved towards every point of the
sky, and to follow the stars from one horizon to the other during their
journey across space.
It had cost more than 400,000 dollars. The first time it was pointed at
the moon the observers felt both curious and uneasy. What would they
discover in the field of this telescope which magnified objects 48,000
times? Populations, flocks of lunar animals, towns, lakes, and oceans?
No, nothing that science was not already acquainted with, and upon all
points of her disc the volcanic nature of the moon could be determined
with absolute precision.
But the telescope of the Rocky Mountains, before being used by the Gun
Club, rendered immense services to astronomy. Thanks to its power of
penetration, the depths of the sky were explored to their utmost limits,
the apparent diameter of a great number of stars could be rigorously
measured, and Mr. Clarke, of the Cambridge staff, resolved the Crab
nebula in Taurus, which Lord Rosse's reflector had never been able to
do.