5-foot reflector. But the first glass disc ordered from
France for the purpose proved radically defective. When figured,
polished, and silvered, towards the close of 1888, it gave elliptical
instead of circular star-images.[1631] A new one had to be procured, and
was ready for astronomical use in 1891. The satisfactory nature of its
performance is vouched for by the observations made with it upon
Jupiter's new satellite in December, 1892. This instrument, to which a
Newtonian form has been given, had no rival in respect of
light-concentration at the time when it was built. It has now two--the
Paris 50-inch refractor and the Yerkes 5-foot reflector.

It is, however, in the construction of refracting telescopes that the
most conspicuous advances have recently been made. The Harvard College
15-inch achromatic was mounted and ready for work in June, 1847. A
similar instrument had already for some years been in its place at
Pulkowa. It was long before the possibility of surpassing these
masterpieces of German skill presented itself to any optician. For
fifteen years it seemed as if a line had been drawn just there. It was
first transgressed in America. A portrait-painter of Cambridgeport,
Massachusetts, named Alvan Clark, had for some time amused his leisure
with grinding lenses, the singular excellence of which was discovered in
England by Mr. Dawes in 1853.[1632] Seven years passed, and then an
order came from the University of Mississippi for an object-glass of the
unexampled size of eighteen inches. An experimental glance through it to
test its definition resulted, as we have seen, in the detection of the
companion of Sirius, January 31, 1862. It never reached its destination
in the South. War troubles supervened, and it was eventually sent to
Chicago, where it served Professor Hough in his investigations of
Jupiter, and Mr. Burnham in his scrutiny of double stars.

The next step was an even longer one, and it was again taken by a
self-taught optician, Thomas Cooke, the son of a shoemaker at
Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of Gateshead
ordered from him in 1863 a 25-inch object-glass. It was finished early
in 1868, but at the cost of shortening the life of its maker, who died
October 19, 1868, before the giant refractor he had toiled at for five
years was completely mounted. This instrument, the fine qualities of
which had long been neutralized by an unfavourable situation, was
presented by Mr. N