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