ible
to see these different images clearly at the same time, because each of
them will render all the others indistinct.

The achromatic object-glass, invented by Dollond, about 1750, obviates
this difficulty, and brings all the rays to nearly the same focus.
Nearly every one interested in the subject is aware that this
object-glass is composed of two lenses--a concave one of flint-glass
and a convex one of crown-glass, the latter being on the side towards
the object. This is the one vital part of the telescope, the
construction of which involves the greatest difficulty. Once in
possession of a perfect object-glass, the rest of the telescope is a
matter of little more than constructive skill which there is no
difficulty in commanding.

The construction of the object-glass requires two completely distinct
processes: the making of the rough glass, which is the work of the
glass-maker; and the grinding and polishing into shape, which is the
work of the optician. The ordinary glass of commerce will not answer
the purpose of the telescope at all, because it is not sufficiently
clear and homogeneous. OPTICAL GLASS, as it is called, must be made of
materials selected and purified with the greatest care, and worked in a
more elaborate manner than is necessary in any other kind of glass. In
the time of Dollond it was found scarcely possible to make good disks
of flint-glass more than three or four inches in diameter. Early in the
present century, Guinand, of Switzerland, invented a process by which
disks of much larger size could be produced. In conjunction with the
celebrated Fraunhofer he made disks of nine or ten inches in diameter,
which were employed by his colaborer in constructing the telescopes
which were so famous in their time. He was long supposed to be in
possession of some secret method of avoiding the difficulties which his
predecessors had met. It is now believed that this secret, if one it
was, consisted principally in the constant stirring of the molten glass
during the process of manufacture. However this may be, it is a curious
historical fact that the most successful makers of these great disks of
glass have either been of the family of Guinand, or successors, in the
management of the family firm. It was Feil, a son-in-law or near
relative, who made the glass from which Clark fabricated the lenses of
the great telescope of the Lick Observatory. His successor, Mantois, of
Paris, carried the art to a point of