Newton's trouble with refractors, chromatic aberration, remained
insurmountable until John Dollond (born 1706, died 1761), after many
experiments, found out how to make an achromatic lens out of two
lenses--one of crown glass, the other of flint glass--to destroy the
colour, in a way originally suggested by Euler. He soon acquired a
great reputation for his telescopes of moderate size; but there was a
difficulty in making flint-glass lenses of large size. The first
actual inventor and constructor of an achromatic telescope was Chester
Moor Hall, who was not in trade, and did not patent it. Towards the
close of the eighteenth century a Swiss named Guinand at last
succeeded in producing larger flint-glass discs free from
striae. Frauenhofer, of Munich, took him up in 1805, and soon
produced, among others, Struve's Dorpat refractor of 9.9 inches
diameter and 13.5 feet focal length, and another, of 12 inches
diameter and 18 feet focal length, for Lamont, of Munich.

In the nineteenth century gigantic _reflectors_ have been
made. Lassel's 2-foot reflector, made by himself, did much good work,
and discovered four new satellites. But Lord Rosse's 6-foot
reflector, 54 feet focal length, constructed in 1845, is still the
largest ever made. The imperfections of our atmosphere are against
the use of such large apertures, unless it be on high mountains.
During the last half century excellent specula have been made of
silvered glass, and Dr. Common's 5-foot speculum (removed, since his
death, to Harvard) has done excellent work. Then there are the 5-foot
Yerkes reflector at Chicago, and the 4-foot by Grubb at Melbourne.

Passing now from these large reflectors to refractors, further
improvements have been made in the manufacture of glass by Chance, of
Birmingham, Feil and Mantois, of Paris, and Schott, of Jena; while