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Page 40
It is difficult to apprehend the tremendous strides which we have made
in the production of telescopes and the consequent increase in our
sweep of the heavens. It was only in 1774 that the elder Herschel
began his work in the construction of reflecting telescopes. These he
gradually increased in size, until near the close of the century, when
he produced an instrument which magnified two hundred and twenty-seven
diameters. In the course of his career he built two hundred
telescopes, having a seven-foot focus; 150 of ten feet and about
eighty of twenty feet each.
With these instruments the astronomical work in the last quarter of
the eighteenth century was mostly performed. The study of the heavens
at this epoch began to reach out from the planetary system to the
fixed stars. In this work Herschel led the way. The planet Uranus at
first bore the name of Herschel, from its discoverer. Sir John
Herschel, son of Sir William, was born in 1792. All of his
astronomical work was accomplished in our century. Following the line
of his father, he used the reflecting telescope, and it was an
instrument of this kind that he took to his observatory at the Cape of
Good Hope. Lord Rosse was born in the year 1800. Under his auspices
the reflecting telescope reached its maximum of power and usefulness.
His great reflector, built in his own grounds at Birr Castle, Ireland,
was finished in 1844. This instrument was the marvel of that epoch. It
had a focal distance of fifty-three feet, and an aperture of six feet.
With this great telescope its master reached out into the region of
the nebul�, and began the real work of exploring the sidereal heavens.
In the reflecting telescope, however, there are necessary limitations.
Before the middle of this century, it was known that the future of
astronomy depended upon the refracting lens, and not on the speculum.
The latter, in the hands of the two Herschels and Rosse, had reached
its utmost limits--as is shown by the fact that to this day the Rosse
telescope is the largest of its kind in the world.
Meanwhile the production of refracting telescopes made but slow
progress. As late as 1836 the largest instrument of this kind in the
world was the eleven-inch telescope of the observatory at Munich. The
next in importance was a nine and a half-inch instrument at Dorpat, in
Russia. This was the telescope through which the astronomer Struve
made his earlier studies and discoveries. His field of observation was
for the most part the fixed and double stars. At this time the largest
instrument in the United States was the five-inch refractor of Yale
College. Soon afterward, namely, in 1840, the observatory at
Philadelphia was supplied with a six-inch refracting telescope from
Munich.
German makers were now in the lead, and it was not long until a Munich
instrument having a lens of eleven inches diameter was imported for
the Mitchell Observatory on Mount Adams, overlooking Cincinnati. About
the same time a similar instrument of nine and a half inches aperture
was imported for the National Observatory at Washington. To this
period also belongs the construction of the Cambridge Observatory,
with its fifteen-inch refracting telescope. Another of the same size
was produced for the Royal Observatory at Pulkova, Russia. This was in
1839; and that instrument and the telescope at Cambridge were then the
largest of their kind in the world.
The history of the telescope-making in America properly begins with
Alvan Clark, Sr., of Cambridgeport, Massachusetts. It was in 1846 that
he produced his first telescope. Of this he made the lens, and such
was the excellence of his work that he soon became famous, to the
degree that the importation of foreign telescopes virtually ceased in
the United States. Nor was it long until foreign orders began to
arrive for the refracting lenses of Alvan Clark & Sons. The fame of
this firm went out through all the world, and by the beginning of the
last quarter of the century the Clark instruments were regarded as the
finest ever produced.
We cannot here refer to more than a few of the principal products of
Clark & Sons. Gradually they extended the width of their lenses,
gaining with each increase of diameter a rapidly increasing power of
penetration. At last they produced for the Royal Observatory of
Pulkova a twenty-seven-inch objective, which was, down to the early
eighties, the master work of its kind in the world. It was in the
grinding and polishing of their lenses that the Clarks surpassed all
men. In the production of the glass castings for the lenses, the
French have remained the masters. At the glass foundry of Mantois, of
Paris, the finest and largest discs ever produced in the world are
cast. But after the castings are made they are sent to America, to be
made into those wonderful objectives which constitute the glory of the
apparatus upon which the New Astronomy relies for its achievements.
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