perfection never before
approached. The transparency and uniformity of his disks as well as the
great size to which he was able to carry them would suggest that he and
his successors have out-distanced all competitors in the process. He it
was who made the great 40-inch lens for the Yerkes Observatory.

As optical glass is now made, the material is constantly stirred with
an iron rod during all the time it is melting in the furnace, and after
it has begun to cool, until it becomes so stiff that the stirring has
to cease. It is then placed, pot and all, in the annealing furnace,
where it is kept nearly at a melting heat for three weeks or more,
according to the size of the pot. When the furnace has cooled off, the
glass is taken out, and the pot is broken from around it, leaving only
the central mass of glass. Having such a mass, there is no trouble in
breaking it up into pieces of all desirable purity, and sufficiently
large for moderate-sized telescopes. But when a great telescope of two
feet aperture or upward is to be constructed, very delicate and
laborious operations have to be undertaken. The outside of the glass
has first to be chipped off, because it is filled with impurities from
the material of the pot itself. But this is not all. Veins of unequal
density are always found extending through the interior of the mass, no
way of avoiding them having yet been discovered. They are supposed to
arise from the materials of the pot and stirring rod, which become
mixed in with the glass in consequence of the intense heat to which all
are subjected. These veins must, so far as possible, be ground or
chipped out with the greatest care. The glass is then melted again,
pressed into a flat disk, and once more put into the annealing oven. In
fact, the operation of annealing must be repeated every time the glass
is melted. When cooled, it is again examined for veins, of which great
numbers are sure to be found. The problem now is to remove these by
cutting and grinding without either breaking the glass in two or
cutting a hole through it. If the parts of the glass are once
separated, they can never be joined without producing a bad scar at the
point of junction. So long, however, as the surface is unbroken, the
interior parts of the glass can be changed in form to any extent.
Having ground out the veins as far as possible, the glass is to be
again melted, and moulded into proper shape. In this mould great care
must be taken to