have no folding of the surface. Imagining the latter
to be a sort of skin enclosing the melted glass inside, it must be
raised up wherever the glass is thinnest, and the latter allowed to
slowly run together beneath it.

[Illustration with caption: THE GLASS DISK.]

If the disk is of flint, all the veins must be ground out on the first
or second trial, because after two or three mouldings the glass will
lose its transparency. A crown disk may, however, be melted a number of
times without serious injury. In many cases--perhaps the majority--the
artisan finds that after all his months of labor he cannot perfectly
clear his glass of the noxious veins, and he has to break it up into
smaller pieces. When he finally succeeds, the disk has the form of a
thin grindstone two feet or upward in diameter, according to the size
of the telescope to be made, and from two to three inches in thickness.
The glass is then ready for the optician.

[Illustration with caption: THE OPTICIAN'S TOOL.]

The first process to be performed by the optician is to grind the glass
into the shape of a lens with perfectly spherical surfaces. The convex
surface must be ground in a saucer-shaped tool of corresponding form.
It is impossible to make a tool perfectly spherical in the first place,
but success may be secured on the geometrical principle that two
surfaces cannot fit each other in all positions unless both are
perfectly spherical. The tool of the optician is a very simple affair,
being nothing more than a plate of iron somewhat larger, perhaps a
fourth, than the lens to be ground to the corresponding curvature. In
order to insure its changing to fit the glass, it is covered on the
interior with a coating of pitch from an eighth to a quarter of an inch
thick. This material is admirably adapted to the purpose because it
gives way certainly, though very slowly, to the pressure of the glass.
In order that it may have room to change its form, grooves are cut
through it in both directions, so as to leave it in the form of
squares, like those on a chess-board.

[Illustration with caption: THE OPTICIAN'S TOOL.]

It is then sprinkled over with rouge, moistened with water, and gently
warmed. The roughly ground lens is then placed upon it, and moved from
side to side. The direction of the motion is slightly changed with
every stroke, so that after a dozen or so of strokes the lines of
motion will lie in every direction on the tool. This change of
di